How does this compare to OTL? I'd guess better, but I don't know enough to really have any proof.
This based on my own understanding of the battle.
@Lascaris or someone else with a more detailed understanding can correct me if they see any mistakes:
(
https://military.wikia.org/wiki/Battle_of_Greece)
As you can see from this map the Germans attacked Eastern Thrace with their Second Army. In spite of ferocious resistance from the Greeks in many of the fortresses further east the Germans were able to push past the Greeks in the West and make it to Thessalonica by 9 April, meaning the 4 and a half divisions of the East Macedonian Army Detachment were out of the picture. This left the Commonwealth W Force on the Vermion-Olympus line facing Second Army directly.
(
https://weaponsandwarfare.com/2019/...g-the-battle-of-vevi-10-13-april-1941-part-i/)
At the same time, 40 Korps which " included two panzer divisions, the mechanised SS Leibstandarte Brigade, and the 72 Infantrie Division" was sent to push through Southern Yugoslavia and outflank the Commonwealth line by pushing through the Monastir gap into the Florina valley in Northern Greece. They were there by 8 April.
(
You can see the mountainous line W force was trying to hold and the valley behind it that the Germans got into through the Florina Gap)
British intelligence had anticipated the plan, and Wilson sent an add hock blocking force to hold them at the mouth of the Klidi pass just south of Vevi, called Mackay force after its commander. This was just meant to slow the Germans down. In the mean time the Greeks divisions of the Central Macedonian Army Detachment (12th and 20th Division, C.M.A on the above map) and W force would Fall back and take up positions on the Aliakmon River, anchored on Mount Olympus in the East. AFAIK IOTL there was no harassment of the German forces as they travelled south. The first Contact was on 11 April against static Australian and Greek positions. There was an armoured formation in the Mackay force but it was used to screen the Northern Kozani Valley (further south) while demolitions were set before being put in reserve with Mackays force. Its infantry and artillery forces were stripped to be added to the static forces near Vevi. At least one squadron was apparently later used to investigate a false alarm of German tanks outflanking the defenses, and succeeded only in losing 6 tanks to broken tracks.
The Germans took several important points from the defense on 12 April, often by using Stug IIIs to climb to areas that neither side had thought accessible and shelling the allied positions. This, combined with infantry and artillery coordination, forced the Australian and Greek forces to evacuate by end of day 12 April in most cases. Many of the units had been heavily whittled down and some of them had even thrown away their light weapons to retreat faster.
I have seen it referenced that Mackay had orders to pull back at 1730 on 12 April anyway, but perhaps that was just to a position further back. As
@Lascaris pointed out earlier, the Greek divisions retreating to defensive positions further in the rear seemed to be expecting Mackay's force to buy them more time, at any rate, though I don't know if this was agreed to or simply expected. Regardless, the 12th and 20th Greek divisions lost a lot of their strength caught before they could make it to their next defensive line and the Germans were able to cut the line of retreat for the West Macedonian Army Detatchment (W.M.A in the map above). This formation then had to retreat in Epirus along with the Epirus army which slowed down the (already pretty glacial) movement of Greek troops south from Albania.
ITTL there seems to be the following advantages over OTL:
1. W force seems to already be deployed along their Aliakmon river line, though I am not sure if this applies to C.M.A as well.
2. The Cavalry forces are better armed (I believe they had Marmon-Harrington's IOTL)and are harassing the Germans well before they reach the Vevi defensive line, increasing German casualties and giving the Allies advance warning
3. The Allies were fairly nervous about facing the Germans IOTL. Apparently on the night of 11 April/Morning of 12 April, any sound or movement in the area was met with artillery fire. One battery apparently spent much of the night shelling an empty hillside because of the report of German tanks. With more success against the Axis in General and the Germans in particular during earlier engagements this tendency may be lessened.
4. If the cruiser tank units are better able to deploy in support they may be able to catch the Pz1's and Stug III's that were apparently the Germans only armoured support and do some damage at some point. Those armoured units were useful to the Germans later, so taking them out of commission as early as possible is definitely helpful.
5. The Germans are a day behind their OTL schedule. This means at least another day for the W.M.A to withdraw from Albania. In addition to possibly saving 12th and 20th divisions of the C.M.A to allow them to withdraw along with the British
@Lascaris has already mentioned that the Cavalry division and possibly a good part of 13th division of the W.M.A may also be able to get out as well. Possibly more depending on how long the Germans can be delayed. This gives the Greeks more forces for the fighting in Central Greece, and may marginally speed up the retreat in Epirus.