@Polish Eagle
We need to consider your scenarios separately, here is the first:
I'd love to see a slightly more detailed discussion of the plausibility of the Greeks holding the Peloponnese in the face of Italo-German invasion. That aside, there is high potential with the quoted scenarios for the situation to end up as Ian W., with the British bottlenecked for a long time and not outracing the Soviets to Romania or Hungary.
Reasons: Germans will know this frontier bears watching and defending, Germans have substantial force to contain the British and take maximum advantage or rough terrain. Germans are there to stiffen the Bulgarians and their Greek, Serb and Albanian puppets.
I agree, it would be difficult for the Allies to decisively push the Axis out of Greece and into the Balkans much more quickly than IOTL. Still, if they do push into Romania, or start hitting Ploesti sooner from airfields in Greece, they might damage the German war effort more severely sooner.
Your second scenario scenario I believe has more potential for a "grand realignment" of the Balkans occurring quickly.
1942 is too early for the British to make any headway and too early for the Greeks to abandon neutrality.
Now 1943 can be a different story. Once the Allies have taken North Africa and cleared Mediterranean shipping routes (with or without seizing Sicily) they can be more convincing when they try to persuade the Greeks they can get forces in fast.
The logistics and infrastructure are not much better, but if plotting can be done between Hungarian and Romanian politicians (and maybe Italian too) within the Axis, the Western Allies and key authorities in neutral Greece and Yugoslavia, the Allies can propose that they can send light but capable forces to aid Hungary and Romania when they defect by being granted unopposed movement through Greece and Yugoslavia and Greek and Yugoslavian reinforcements.
Neither neutral Greece, nor neutral Yugoslavia will easily jump in the war if they think that will make them a battleground or a front-line.
If they think there is an understanding with Hungary and Romania where the latter defect though, and that western troops will pass all the way through them, then Athens and Belgrade could see such cooperation as improving their security for the wartime and post-war. Germany is going to start looking weaker after Stalingrad.
Even if the Western Allies only have a crappy transportation network and rugged terrain to get forces through to Hungary and Romania, in the absence of German opposition on the spot, with Yugoslav and Greek acquiesce/nonresistance or even collaboration with Allied movement, then the western allies can beat the Soviets to Bucharest and Buda.
I admit, the Balkan front of WWII isn't my strong suit. This thread originated from a "How could Mussolini redeem himself?" discussion, where I suggested that the only way for Mussolini to do that would be to jump ship before Unconditional Surrender becomes an official Allied war aim, and that this would require Allied victories in the Balkans (because an invasion of Italy would render the question of Mussolini changing sides moot) by 1943. The Siege of Malta would definitely have to be resolved much sooner than IOTL--possibly with a heavy infusion of American naval assets to the Mediterranean.
Yugoslavian neutrality is not a scenario I had considered, but it makes sense that it would be required to get the entire Balkan Peninsula on the Allied side--once Hungary goes full "Revise Trianon," they're too heavily invested in the Axis cause for an easy defection.
Yugoslavia's partition was a consequence of Axis pressure on the Balkan states to join the struggle against the Soviet Union--but without the Italian invasion of Greece, there's less work for the Axis to do at all in the Balkans, so maybe Yugoslavia could avoid German and Italian interest.
According to Wikipedia, the Germans opposed the invasion of Greece in the first place, preferring Italy to concentrate on North Africa. Maybe if Mussolini pays more attention to German warnings and does so, the Balkan front can be avoided and the Balkan realignment you propose can go ahead in 1943. By that point, the Soviet counteroffensive puts the specter of Communism back into Europe, so the threat of Soviet domination would be an incentive for the Germans' Balkan allies to defect.
If the Allies play their cards right, they can get troops all the way up along the Carpathians and moving on Vienna by the start of 1944. And cut off from Romanian oil, the German war machine would collapse in Ukraine and Belarus more quickly.
Of course, all this is contingent on Greece deciding to join the Allies in 1943, as opposed to following the Turkish example of remaining neutral until the last moment. Unlike the Hungarians and Romanians, the Greeks have no reason to believe that the Allies would allow the Soviets any influence in Greece--so what could the Allies promise them to get them to join, if the Germans and Italians are distracted in Russia and North Africa?