I suspect they might only need to send one armoured division, since such mountainous country isn't terribly suitable for massed armour. The other caveat about this is that half the infantry would be Australian, that might cause some political difficulties eventually. It won't be long before the Australian government starts calling for some of these troops back.
Due to the OTL, I doubt it would be a problem. In OTL, the 7th Australian Division was earmarked to arrive in the second wave along with the Polish Brigade. And it would be sent with the position in Cyrenaica collapsing while a hostile Vichy was on the eastern flank. Now that there is no North African campaign, I expect the 7th to arrive with the first wave.
Without having a North African Front, the Australian government won't object in sending the 9th Division in a second wave. They may object in sending the division to the frontline though. So, it can be form a mobile reserve in the Thessalian Plain behind the frontlines. After all, in OTL the Australians were fine having all of their divisions committed. In TTL the Australian committment will be lighter- in OTL you had the 6th in Greece and Crete, 7th in Syria-Lebanon and the 9th under siege in Tobruk. In TTL the frontline will be short enough to one Australian Division fully committed in one of the passes, one holding a quiet sector behind a mountain range with minimal fighting and one just camping in the rear.
My question here is whether Algiers would want to keep some of those troops in Syria for internal security.
I don't think they are needed. In OTL they were demobilized after the Armistice (and they the hostile British in Palestine in addition), so they were not needed for internal security. As I see it, the divisions had been formed to land in Thrace, as Weygand was suggesting. So, they would be used in their intended role.
If the 3 divisions were not needed in OTL with the British next door, while would they be needed now when there are allied British in the south and friendly Turks in the north? There is really no reason not to send them.
Their main factory appears to be in Athens. It's sure to be a priority target for the Luftwaffe. My main worry - as noted above - about the Allied position in this ATL Greece is the air question - the Allied air forces in mainland Greece will, at least initially, lack good early warning (no radar) and strong AA defences. Which gives me an idea...
When we talk about the aerial warfare in the Greek campaign, have in mind that the Germans will gain superiority (until June) but not air supremacy.
In April 1941 the Luftwaffe sent 1,000-1,200 aircraft against both Yugoslavia and Greece. They used airfields from both Austria and their satelites. I doubt they can send 1,000 planes against Greece in the short term. The bulgarian airfields had been expanding for more than a month to host the aircraft they did in OTL. When the German capture northern Greece they will obtain only a single airfield in Sedes, Thessaloniki. To throw the full 1,200 aircraft in a sustained effort against the Allies in Greece they will need more time expanding the existing airfields even more (in Bulgaria they were operating in full capacity to my knowledge) and build more airfields. This takes time and the window of opportunity last until the final week of May- as you mention at late May they vanish. So, how many aircraft can they throw against Greece in April-May? I don't know but certainly quite fewer than 1,200.
Now the British don't have an active campaign in Western Desert. Moreover, they don't have an actual threat against Alexandria and Suez since the Italians in the Dodecanese are cut off from supply for months. At the
very least, the RAF can deploy 2 more fighter squadrons and 1 more Blenheim squadron in Greece with the First Wave. As they continue building up their bases in Attica and Crete, they can also move the 3 Wellignton squadrons in late April. By the end of May/ June they can add 1 RAAF fighter squadron flying P-40s, 1 Beaufighter squadron (to cover the supply route from Crete- they will have to deal with bombers not Me-109s) and 4 squadrons coming from the East African Front.
I guess the French would want to both deploy some squadrons of the AdA in Greece but also to conserve their limited pilot pool. Their main bottleneck is aircrew and not machines after all. How many aircrew would be sent? If I had to make a guess the French can deploy as many aircrew as they had in Syria-Lebanon in 1941. If the Vichy would sacrifice these aircrew in a backwater and not in North Africa, the French in TTL can do as well.
Lastly, the British in TTL have no reason to keep the greek order of 30 P-40s for themselves. So, the Greek Air Force will have 30 more modern fighters compared to OTL - when they were basically out of machines.
Overall, the Germans will have the upper hand but they will face much stiffer resistance compared to OTL. Any german aircrew shot down behind the frontlines will be POW. The Allied aircrews that are in a defending role, if they are shot down and survive, they will get wine and tsipouro from the locals and return to their airfields shaken but buzzed.
Can they retreat in good order to the Olympus line?
If they have even a single additional week without the Allied front collapsing, then absolutely.
What are the possibilities for transferring German forces to the Albanian front and attacking on the Ioannina - Arta axis?
Extremely few- their supply will have to come down the whole italian peninsula and then shipped to Valona. And the albanian ports operate in full capacity trying to supply the existing italian formations. If they actual try to do something like that (which I doubt) it would be such a clusterfuck with the logistics that it would be a major Allied boon.
The scenario I envisage for Greece therefore goes something like:
1. The Allies get enough troops into the line the prevent the Germans from breaking through the Olympus line in April. There is then a brief pause while the buildup goes on.
2. Major German effort in May, involving their airborne troops also. They are under instructions to break off the offensive if victory is not achieved by the end of May.
3. The Allies hold on. At the end of May the Luftwaffe, despite having gained local air superiority, mysteriously vanishes, as do the German Panzer forces.
4. June sees no more than local actions. Barbarossa happens on schedule.
5. July: the Allies mount a major offensive, as a political offering to encourage Stalin. It fails too - the Axis are also very well dug in. Both sides settle in for the long haul. Heavy Australian casualties and the growing Japanese threat prompts Canberra to insist on getting most of its troops back to the East.
6. The Allies begin a push in the Dodecanese, taking Karpathos and Rhodes by the end of the year.
How does this sound?
Well, good overall, but a few notes:
In OTL the Germans had decided to use a regiment of paratroopers to capture Lemnos island. At the beginning of April a british battalion had landed in Lemnos. Since it is in a strategic spot, I believe the British would have the opportunity to send more troops there instead of a single battalion. Perhaps the 22nd Brigade that is not needed in Western Desert. If the Germans use just the single regiment they wanted in OTL, then the British will have the upper hand. If they throw the full division, then the Germans have the upper hand. If the Germans try to drop paratroopers both in Lemnos and behind the Olympus Line.... well that would be the best for the Allies.
I don't think the Allies will have the ability to begin a major attack in July. A tactical attack in a limited front as a show of support - e.g. seize a few hills and better secure a pass, then yes. Their immediate target will be the Dodecanese Islands- the RN will demand it as they threaten the supply lines. Their first priority in OTL planning was to be defensive in the mainland and attack the islands. If I may make a suggestion, I find it more plausible as a show of support to engage in a limited attack in the mainland while they seize the Dodecanese in summer 1941.