A couple of more thoughts:
1) Constantine agreeing to join the war in 1915 would not have exactly resolved the national schism, but would have gone a long ways towards minimizing it.
2) I think that If I were Greece, I'd leave the French, British, Serbs, and a third-rate Greek corps to it in Salonika and focus on Albania. The Greeks were already in North Epirus until driven out by the Italians in 1916. They could have had a corps or two, as an official ally, engage the Austro-Hungarians in a semi-independent campaign, and held most of the country by the end of the war.
Now, imagine this: Greece and Yugoslavia divide Albania, as was discussed from time to time. The great powers, war weary, accept it. The Italians are pissed, and maybe they're in Vlore and maybe they're not, but in any event they can't do much about it.
Could Italy have invaded Greece in 1940 without a base in Albania? If not, would British troops been in Greece? If not, would Yugoslavia have tried to void the alliance with Germany? And again, if not, would Germany have delayed Barbarosa by 6 weeks in order to secure the Balkans? How much more could they accomplished in Russia with another month and a half in 1941?
Interesting questions.
Italy's entry into the two World Wars was tragic beyond belief. It gained them nothing, cost them a tremendous amount and left the nation less well off than it had been. The enormous losses of WWI, which gained them far less than they had been led to believe they would gain from entering it on the Entente side embittered a large segment of the populace and made them susceptible to the demagoguery of the Fascists. The nation would have been better off staying neutral or even joining the CP IMO.
Would Italy have been able to launch the invasion of Greece in 1940 without Albania? No. IOTL it was a toss up for Mussolini whether to invade Yugoslavia or Greece, Hitler didn't want Yugoslavia invaded (he later had to reverse that decision after the coup) so it became Greece. So if he invades anyone, it'll be Yugoslavia, as he has a land border there. Italy being shut out of Albania MIGHT have led to even more investment and immigration to Libya, which would pay enormous dividends for Italy later, perhaps even keeping them out of WWII.
No Italian Albania means there will probably be no German invasion of Greece. Note I said probably, as it isn't impossible that Greece would feel the need to help Yugoslavia if the Germans attack it. I think it's possible both nations stay neutral, they will certainly at least try. Whether Adolf feels their presence is acceptable is open to debate, IMO he will leave them alone and concentrate on the Soviets. I'll let others more knowledgeable debate how not having to invade Yugoslavia and Greece affects the timetable of Barbarossa.