Thorondor as a Maia???

[the phantom]

If we suppose such a union could beget offspring, it is still questionable whether these would be raised to about the same level as their parents, or whether they would fall under the category of beasts raised to a higher level, in which case one would have to suppose that the union would have violated an axan anyway.

Well said. You've explained part of the problem I have with an Ainu-Ainu offspring.

but if it is possible for a Maia in incarnate form to mate with a non-Maiar being to produce incarnate non-Maiar offspring, why should it be impossible for a non-Maia Gwaihir to be descended from a Maiar but incarnate Thorondir?

Sharku answered that pretty well with this-

While it was very likely the case that Ainur spirits conceived children with non-rational beings (without a fea), that would have to be attributed to the bad side only.

In other words, can you imagine Thorondor (a good guy) having an offspring with a common animal?

Tolkien would certainly not allow such behavior on the good guy side.