Was Alexander an ethnic greek?

I pretty much agree with what you have said.

My only concern is that you use language as the only criteria of ancestral links when you say that Albanians and Greeks are ancestors of their ancient equivalents, but Macedonians and Bulgarians are not.

For example are the natives of the Andies,  many of whom have adopted the Spanish language and have been assimilated into spanish culture, not decendants of the ancient Incas?

But there are many more sources:

Demosthenes:

"not only no Greek, nor related to the Greeks, but not even a barbarian from any place that can be named with honors, but a pestilent knave from Macedonia, whence it was never yet possible to buy a decent slave" (Demosthenes, Third Philippic, 31).

Arrian:

"Greek mercenaries attacked the Macedonian phalanx� the Macedonian centre did not set to with equal impetus� and the Greeks attacked where they saw that the phalanx had been particularly torn apart. There the action was severe, the Greeks tried to push off the Macedonians into the river and to reserve victory to their own side� There was also some emulation between antagonists of the Greek and Macedonian races" (Arrian 2.10.4-7).

These are riduclous assertions. The Byzantine empire, like most empires, was not an ethnically homogenous one, and although it was dominated by Greeks, Greeks were by far not the only ethnic group in that empire. The Byzantine Empire was a multiethnic empire, which encompassed territories that went far beyond than those that were traditionally inhabitted by the greeks, it was a multiethnic empire, and it united its subjects not around the concept of greekness, but around that of orthodox christianity.

http://www.historyofmacedonia.org/RomanMacedonia/ByzantineEm pire.html

Is this just a guess?

By the way talking of paternal line all the back to Alexander, could you clarify for us how many children did Alexander actually have?

This would be a ridiculous claim. Only nationalists from the Republic of Macedonia would make such claims. Alexander of Macedon was certainly not a slav, and certainly he was not a greek either. The macedons were a distinct people that have been gradually assimilated by the greeks and the slavs that later came to the region.