If Mussolini hadn't invaded Greece and totally screwed up the Balkans Germany may not have had to help them. Then perhaps the Soviet invasion would have gone off on schedule in May instead of late June. Four to six weeks of fair weather may have made a difference. And the Soviet divisions would have been in that much worse shape.
iirc, the late Spring rains in 1941 would have delayed the start of Barbarossa until mid June anyway. Perhaps it could have started 1 or 2 (at most) weeks earlier, but it has been 30 years since I did that research so I am not 100% positive.
I don't see where a neutral Greece means the Allies land and then move up towards Berlin. Exactly which country are they going to go through and you have to realize the terrain whichever way they go is far tougher than trying to go across the plains of northern France to reach Germany. In addition, how exactly is an invasion staged out of Greece supposed to succeed when the lines of communication are very exposed to Axis countermeasures?
Now granted a scenario could be that the British are "welcomed" into Greece, set up shop, build airbases and have at it with Germany's oil supplies, but I don't see this as a very likely scenario given the Greeks would likely be very concerned about retaliation from the Axis powers in terms of air power, etc. In addition, there is a very good chance that the German paratroopers would have been used to take out Malta, giving the British no flank protection from the Italian mainland and also increase the issues of trying to detect if the Italian fleet is at sea or not. The British (and Americans) just would not have the ability to project the type of power necessary into the eastern Med before 1943 at the earliest and then if it became a choice between landing in a neutral Greece or Italy/Sicily, they would have likely gone with Italy/Sicily again.
