1. That's a really good point. Early Greek participation would hamper A-H in Serbia. They do have Bulgaria on their side, which will help somewhat. The whole thing will probably be an analogue to Italy's participation IOTL. If this is the case then Russia will still do well in Galicia and A-H participation on the eastern front will be unchanged. However, if France is still defeated, none of it will matter when the Germans come knocking with their entire army (or at least most of it if they send aid to A-H). As for Greek participation at Gallipoli, since the campaign was a miserable failure IOTL, I see it in this timeline being, instead of pushed right back into the sea, degenerated into a fruitless drain on entente resources (especially for the Greeks, who will have fewer men to bear against Austro-Bulgarian forces) while the Ottomans wage guerrilla warfare. It may also save Ottoman resources that were wasted in the debacle of the Caucasus campaigns.
2. Italy could probably be persuaded with a German promise of big French colonies (maybe offering to back Italy up in demands for Tunisia and Algeria) and maybe a monetary award similar to the one given to the Ottomans. Its navy will perform poorly, even with help from the Austro-Hungarian navy, but this (and French troops having to be diverted to Libya) will still mean that entente resources must be expended on them. Germany, via A-H will probably be able to funnel in some supplies. Also, it might not ever get to matter if the French are defeated quickly enough (say, if Italy happens to join just prior to Ypres).
I can't argue much with your points, except, maybe about the Greek participation in the Galipoli campaign:
The Anglo-French force failed (among other reasons) because of 1. faulty maps, 2. Lack of experience in forced landings, 3. lack of experienced troops in modern warfare, 4. delay in order to convince Greece to participate and make up the invasion plans, which gave the Turks and their German advisers time to prepare.
But with Greece participating: 1. Much better maps, 2. Experienced navy and army in landings, 3. Troops with a lot of experience facing machine guns and fortified areas, 4. faster action (troops gathered a lot faster and use of already made invasion plans), hence less time for the Turks to fix their lines and gather ammunition.
Now, concerning the rest, we should check time and numbers:
1. When does Italy and Bulgaria join CP?
2. How many French divisions are required to secure the mountainous borderline with Italy?
3. How many troops does France save in the event of a Macedonian Front maintained mainly by Serbians and Greeks?
4. What effect would the survival of Serbia in 1915 have on the Russian strategy? Is there a possibility to press more through the Carpathians, puting A-H in worse position, leading to larger contribution of Germany in the Eastern Front?
5. What happens with Romania? Would she join Entente earlier? Does she collapse like IOTL, or survives and ties down more A-H troops in Transilvania while diverts some of Bulgaria's attention, even affects the Bulgarian stance if that happens early enough?