Frankly, I don't see how the retreat from Greece could plausibly be any worse than in OTL.
If the West Macedonia Army Section is able to retreat, then the mountain passes will be covered with worn-out formations that nevertheless are experts in mountain warfare. That know how to use and hide each mountain gun battery, how to storm positions in the mountains, how to set up good defensive positions in a matter of hours.
Beyond literature sources, I have had another source on the above: my grandfather was a soldier of the XIII division that fought the Leibstandarte SS Adolf in Lake Kastoria. Their morale was high despite the casualties and the retreat. When there was a gap in the defence, the mule drivers rushed to cover it. A unit with its morale broken wouldn't have its rear echelon personnel to grab rifles and rush against a mechanized force.
If the XIII Division was
attacking at April 8th in
Kalivac and by April 15th had managed to retreat close to
Argos Orestiko and units of the division fought well against the Leibstandarte (as Lascaris mentioned, Dietl thought he was fighting 3 divisions, not 3 understrength battalions) then even a single more day bought by the 1st AB tanks would be enough. Frankly, since the panzers met
much stiffer resistance TTL, they would need another day to rest and repair before continuing the advance. So, 48h hours at least, even if the 1st AB stops fighting any delaying actions at all.
24 or 48 hours make a huge difference. In the span of a week, the XIII -while attacking the Italians at 1500m altitude- retreated successfully a distance of 124km
on foot and still had enough strength to give the SS a hard time. If the XIII could made it, the rest of the Army Section needs a day or two to manage the retreat towards Grevena. Not to mention that the Epirus Army Section will retreat to the Kalamas Line with the V "Cretan" Division rushing forward to get to ports for Crete.
Also in some occasions, I notice a weird trope when it comes to the Greek Campaign. Only the Commonwealth has problems with logistics, but not the Germans. The Commonwealth has internal lines of communications and its main logistics bases are developed ports in Volos and Piraeus. The main logistic bases for the Germans are ... in the Danube. The author described accurately that poor greek roads would be impassable after the first armoured formations pass through. That applies to both the Allies
and the Germans. Even worse, the Germans would have to advance through the churned-up mud left behind by the retreating Commonwealth. Even more importantly, with even just 24h window, the Commonwealth and Greek engineers will have time to blow up a great many bridges. And there are only 3 roads in total that the Germans will have to pass through. Every single time, even without armed resistance, the advance has to pause and the german engineers will have to repair the blown up bridge over that difficult terrain.
24h behind OTL schedule at April 15th-16th is the equivalent of compound interest. More bridges blown up, more greek units getting in the Aliakmon Line, more delays. Everything is compounding and butterflies affect the campaign in dozens of small details.
Edit: The Greeks have suffered grievous casualties in the past months against the Italians. Some units are at 50% strength. Others don't have their full artillery park. But in the short term (1-2 weeks) it doesn't mean much if they can retreat behind the Aliakmon Line. Despite having Stukas, artillery and mortars, defensive positions in the mountains need to be stormed with hand grenades and bayonets. The OTL Allies learned that lesson in the Italian Campaign. At this point in the war, the Greeks are the most proficient army in fighting in the mountains with grenades and cold steel. In the short term their bite will be tenacious one in a defensive role in the mountains. In the mid-term the vast majority of the units will be destroyed and only a minority will escape to Thermopylae either by railroad, horse or truck.