Phillip is the kind of guy who's bold enough to at least press into Mesopotamia and definitely try and bag Egypt.
I'd agree with a Philip taking Egypt though...
- before or after an Egyptian conquest.
Well, I was first struck by absurdity of the Macedonians not conquering Egypt.
But actually that is the point of this thread.
That's what differentiate it from hundreds of similar threads.
You gotta try something new from time to time.
So we have to live with that.
And I guess that's doable.
Phillip (or Alexander, under this scenario it's not that important) dies after conquering Anatolia and the Northern Syria. What is important is that there's a good old succession crisis after the death of the Macedon king, whoever he is.
So there are a few pretenders to the throne, some using some figureheads from the old dynasty, some proclaiming themselves Macedon kings. In Macedon and in her new Empire everybody is busy by fighting.
So the Persian Empire and the independent Egypt have some breathing space to consolidate.
At the end of the terrible civil war the Macedonian Empire stays like the topic starter wants - holding Macedon, Greece, Anatolia and the Northern Syria. It doesn't disintegrate because it's not big enough, which is kind of advantage in this sense - compact and manageable.
After severe bloodletting the Macedon Empire is strong enough to hold together but not that strong to continue the conquest of Egypt and the Persian Empire; and soon the Celts would appear from the North to keep the Macedonians busy.
As for Egypt... if they get 30-40 thousand local soldiers trained in a Macedonian style, they'd be a fource to be reckoned with. If the Ptolemaids did it in OTL, why couldn't the native Egyptian dynasty do it in ATL. Because of the Macedonian refugees from their civil war there won't be lack of the Macedonian military instructors in the region willing to get hired.
Getting the locals trained is
much cheaper, so the idea is attractive.
And, ye, let's give Egypt the Southern Syria: every independent Egypt is paranoid about Syria, as it is the natural doorway into Egypt; and keeping your door open is always a bad idea...