The one thing I am sure about is that February 1948 was too late, regardless of what Benes did and regardless of whether the USSR would send troops or not. [1] I'll give a few examples of how overwhelming Communist power was in Czechoslovakia by that time:
(1) The first thing to remember is that the non-Communist parties had largely imprisoned themselves with their cult of the National Front for the previous three years. (For obvious reasons, given Czechoslovakia's history, fear of a future resurgent Germany, not of the Soviet Union, was the chief fear. The National Front and friendship with the USSR were considered the only guarantee against the German menace.) This made the idea of a government without the Communists, who were by far the largest party in the Front, almost unthinkable. Moreover, in each of the non- Communist parties there were some outright stooges of the CPCz, (like the "Social Democrat" Fierlinger
https://en.wikipedia.org/wiki/Zdeněk_Fierlinger) as well as some who took a temporizing attitude during the crisis. This enabled the Communists even after February to maintain the fiction that a multiparty National Front still governed Czechoslovakia--though of course all the non-Communists in it had to be approved by the CPCz.
(2) The non-Communist ministers during the 1948 crisis did not demand a government without Communists; rather, they simply wanted the police to stop dismissing non-Communist officers. The police, already dominated by Communists, refused. When this happened the majority of the non-Communist ministers submitted their resignations. According to the Czechoslovak constitution, when one-half the ministers resigned, the government was dissolved; evidently the non-Communists were counting on triggering that provision, and forcing new elections. (They did not want to do away with the National Front; they just hoped that the position of the non- Communists within the Front would be strengthened by the elections.) However, as it turned out, only twelve of the 26 members of the government resigned; the non-Communists had counted on Jan Masaryk to join them, but he used an attack of the flu as an excuse to do nothing. The Social Democrats also hesitated to resign. So even from a formal/constitutional point of view, the position of the non-Communists was weaker than one might think.
(3) if we look beyond that to control of the streets, it was no contest. I have already mentioned Communist domination of the police/security forces (which was the immediate trigger of the crisis). Also important was the Communist control of the trade unions. Although it was not until February 25 that Benes decided to accept the resignation of the non-Communist ministers, the outcome of the crisis had already been determined. Huge mass meetings were organized by the Communists. On February 21 Gottwald called for the formation of "Action Committees of the National Front." On February 22, almost 8,000 delegates to the Congress of Factory Committees (which nominally consisted of members of all parties as well as non-partisans) voted to back Gottwald's proposals for further nationalizations and for solving the government crisis according to Gottwald's recommendation (i.e., Benes should accept the non-Communist ministers' resignation and replace them with ministers acceptable to Gottwald). They also voted to set up a National Action Committee and to take control of the enterprises to be nationalized. There were exactly
nine dissenters!
To show how hopeless the non-Communists were in the face of the Action Committees' onslaught (which included the seizure of radio stations, the arming of "Workers' Militias", paper mills denying paper to non-Communist papers, non-Communist officials being locked out of their own offices, political party headquarters being occupied and turned over to the most pro-Communist wings of their parties etc.): on February 24, in the general one-hour symbolic strike called by the trade unions, "more than 2.5 million workers participated. About 4,000 wage earners did not strike." Karel Kaplan,
The Short March: the Communist Takeover in Czechoslovakia 1945-48, p. 183. Benes' own chancery observed the strike, as did Jan Masaryk's in the Foreign Ministry.
(4) As for the army, I have already indicated how strong the Communist presence in it was, thanks to General Svoboda. I'll only add the following from Zbynek Zeman and Antonin Klimek,
The Life of Edvard Benes 1884-1948: Czechoslovakia in Peace and War (Oxford: Clarendon Press 1997), p. 267:
"There were other hints that senior officers were not taking Benes' function as their supreme commander seriously enough. General Svoboda attacked those politicians who would not co-operate in the establishment of the new 'action committee' of the National Front. At a public meeting attended by a Red Army general, General Bocek, whose political allegiance was to the National Socialist Party, spoke of the need for the closest possible alliance with the Soviet Union, in a world where imperialist forces contended for primacy with the forces of freedom, progress and people's democracy. The presence of the generals at political meetings, at the height of the crisis, was surprising; the contents of their speeches, on the other hand, were by no means exceptional. They spoke in a way expected of them in the years after the war, as loyal supporters of the National Front; their declarations nevertheless meant that Benes could place no reliance on the armed forces." In short, had Benes not accepted Gottwald's "proposals" (i.e., demands) there would not only have been a bloodbath but one the Communists were almost certain to win, even without direct Soviet intervention. Benes has been criticized, and so have the resigning ministers (their resignations certainly played into Gottwald's hands) but the fact is that as of February 1948 it was just too late to prevent a Communist takeover of Czechoslovakia.
(5) Benes lived long enough to justify his conduct:
"They blame me that I let them down, and I blame them, because they let me down at a decisive moment. I was powerless without the action they promised. When Gottwald filled the Old Town Square with bloodthirsty militia, armed to their teeth, I waited for the rest of them to assemble in Wenceslas Square. But I could not imagine that they so much lacked organization, as well as decisiveness, when they should have acted. I believed that the demonstration of unarmed students would be a signal for a general uprising. But when no one moved, I could not allow Gottwald's militant hordes to carry out mass massacres on the defenseless people of Prague. Their threats had no limits."
Quoted by Zbynek Zeman and Antonin Klimek,
The Life of Edvard Benes 1884- 1948: Czechoslovakia in Peace and War (Oxford: Clarendon Press 1997), p. 268.
And if anyone thinks that Benes was exaggerating the ineffectiveness of the non-Communist leaders during the crisis, Zeman and Klimek note that "At different points in the February crisis, they were not available in Prague. Mgr Hala and other leaders of the People's Party attended a congress of Catholic women; Petr Zenkl, one of the ministers who had resigned, left for a provincial town in Moravia where he was to receive honorary citizenship. The general secretary of the National Socialist Party, [Vladimir] Krajina, also left for Moravia on a lecture tour; and his party comrade, Hubert Ripka, left Prague to visit his parents. The chairman of the Sokol organization went to the Tatra mountains in Slovakia for the opening of the Sokol skiing championships. Jan Masaryk, still a member of the government, was in bed with a cold throughout the crisis, available to no one with the exception of Mr. Zorin [the former Soviet ambassador and now Molotov's deputy]." p. 269.
These people were not without courage and ability. Zenkl had endured years of imprisonment at Dachau and Buchenwald.
https://en.wikipedia.org/wiki/Petr_Zenkl Krajina was a resistance hero who later had a distinguished career as a botanist and forest management specialist in British Columbia where he led a successful campaign to establish more than 100 ecological reserves.
https://www.botany.ubc.ca/people/vladimir-krajina But in February 1948, even if they been better organized, it is not clear there is much they could have done.
***
So if February 1948 was too late, what was the last possible POD? I think that the 1946 election was the key. The Communists did not get the majority they had hoped for, but they were easily the largest single party with about 40 percent of the vote, and they got the Prime Minister's position and the Ministry of the Interior, and--through the "non-party" Minister of Defense Ludvik Svoboda--control of the armed forces. After that IMO a Communist take-over was only a matter of time, and would probably not require Red Army intervention.
How could the 1946 results have been changed? According to Benjamin Frommer, in
National Cleansing: Retribution Against Nazi Collaborators in Postwar Czechoslovakia, p. 216, "The Communists' political competitors retrospectively blamed disfranchisement for their election debacle. They claimed that machinations by partisan national committees unfairly prevented as many as 250,000 to 300,000 alleged collaborators from casting ballots. Although claims that disfranchisement cost the 'democratic' parties at least ten parliamentary mandates are probably exaggerated, had a mere three seats (approximately 72,000 votes) changed hands, the Communists and their thus-far obedient Social Democratic allies would have lost their majority in the new Constituent National Assembly." But Frommer adds that "Even if the numbers are accurate, there is still no reason to assume that all of those denied the vote would have supported non-Marxist parties."
http://books.google.com/books?id=gU-dZ5Zk2-YC&pg=PA216&sig=HsHIYZXGkhQFWJ7jWkhY8kCXMSQ
Had the non-Marxist parties gotten a majority in 1946, they would still form a coalition with the Communists, but they would at least potentially be in a stronger position to demand that the Communists be excluded from some of the key positions (e.g., Interior Ministry) they had in OTL. If the Communists said, "no we must have all the key offices or we won't participate at all," the non-Marxist parties (Benes' National Socialists, the [Catholic] People's party, and the Slovak Democrats) could have formed a government of their own. Would Stalin, who had withdrawn Soviet troops from Czechoslovakia before the election, have sent them back in if the non-Marxist parties refused to yield to the Communists' demands? I doubt it; in 1946, Stalin was probably still not ready for a total break with the West (after all, Communists were still participating in the French and Italian governments, the Marshall Plan and Truman Doctrine were still in the future, etc.) He might tell the Czechoslovak Communists to accept a less dominating role in the government than they got in OTL, in the hope that a take-over was still possible in the future (for example, the Communists still controlled the trade unions, and could call crippling strikes--though in France and Italy this capacity was not sufficient to enable the Communists to gain power).
[1] It was once believed that Stalin had wanted to use Soviet troops, but that Gottwald had assured him that it would not be necessary: "Stalin, he [Zorin] told Gottwald, insisted that the Communists should take advantage of the current crisis to stage the final confrontation. He also pointedly suggested that Gottwald should ask the Soviet government for military assistance; Soviet troops were already massed on the Hungarian border...Gottwald declined to invite in the foreign troops, making this the one instance in his entire life when he disobeyed the Soviet leader. When Zorin reminded him of the consequences of such disobedience, Gottwald explained that the presence of Soviet troops was completely unnecessary because the party was in full command of the situation, with control over the armed forces, and that Soviet intervention would significantly complicate the power confrontation both domestically and internationally." Karel Kaplan,
The Short March: The Communist Takeover in Czechoslovakia 1945-1948 (New York: St. Martin's Press 1988), p. 175. It now appears that this has it exactly reversed--it was Gottwald who wanted to use Soviet troops (at least for a demonstration on the borders) and the Soviets who turned him down!:
"The newly released documents argue against the currently widespread version, which claims that the Kremlin considered it appropriate for the Soviet Army in Hungary to move towards the frontier with Czechoslovakia or even to enter Czechoslovak territory. It is clear from the telegrams, that the Soviet leadership considered the Soviet demonstration of military might at the frontier with Czechoslovakia, which according to Zorin Premier Gottwald and General Secretary Slansky were demanding, to be inappropriate. Molotov also thought it inappropriate for Moscow to give Gottwald any orders 'along Party lines' and warned against Zorin visiting Benes without direct orders from Moscow. The Soviet Communist Party publicly expressed its position on the events in Czechoslovakia with only a commentary in
Pravda, remarking on the international 'reactionaries' and emphasis on the inadmissibility of any compromise.
"The Editors of
Soudobe dejiny have included with Murasko's article the comments of Russian historian Leonid Gibiansky and Czech historian Karel Kaplan. Both scholars point out that even after the publication of part of the documents connected with the Zorin mission a number of open questions remain concerning the evaluation of the Soviet role in the February takeover. The Communists' opponents, including President Benes, had no idea of what decisions had been taken in the Kremlin, and so the CPCz leadership could use the Soviet threat as an instrument of political and psychological pressure. And, Kaplan argues, Moscow had no need to give Gottwald instructions emphasizing a tough, uncompromising approach, as Zorin had originally said about him, when in the meantime it became apparent that the CPCz had become master of the situation."
http://web.archive.org/web/20030709111152/http://www.usd.cas.cz/usdeng/zorin.html