My own knowledge - the Egyptians conception of the Divine changed quite a bit, but (excluding Akhenaten's reign) they remained polytheistic with varying degrees of hardness. One thing that shows that they weren't completely hard was their syncretism of deities, Sobek-Ra, Amun-Ra, Ra-Horakhty and so on.
As already mentioned, the synchronization of deities is evidence enough that the Egyptians were not hard Polytheists. They may have been in the earlier dynasties, but the Faith of Kemet was always a living and evolving thing, and many Kemetics today believe in that spirit of progression.
Also Hornung's book you were referred to above is excellent. I would say that the House of Netjer's teaching that the Egyptians were soft Polytheists is closest to the truth.
I can think of a handful. One is that in Genesis 41 and 2 Chronicles 35, the pharaohs recognize God, in their language Netjer. These passages were written at a time when Egypt was existing as still ancient and right next door to Israel. So the authors would have known whether Egypt thought of God as God or not.
The counterargument is that the Tanakh is biased or could make things up. I can agree a bit, but in this case I think the authors were sincere on this question.
What is the arguement that the Egyptians were hard polytheists?
One argument is that only Israelite religion and maybe a few others like Akhenaten's were the only monotheists, so that means by default everyone else is polytheist, at least when they teach more than one "god."
My own knowledge - the Egyptians conception of the Divine changed quite a bit, but (excluding Akhenaten's reign) they remained polytheistic with varying degrees of hardness. One thing that shows that they weren't completely hard was their syncretism of deities, Sobek-Ra, Amun-Ra, Ra-Horakhty and so on.