[Aiwendil]

I don't have time to respond at length at the moment, but a few points occurred to me while reading your post:

However, in HoMe XII, Tolkien states that (when discussing the Olog-hai) "there was no kinship from the beginning between the stone-trolls and the Orcs so that they might breed together".

There are problems with this statement. Elves and Men, for example, had no kinship from the beginning, but they can breed together.

as HoMe X shows, Tolkien decided that Elves and Dwarves were too "resistant" to be turned into Orks

Not definitively. He seems to have decided in one essay that they must be Men, but later he says something to the effect of "It is probable, then, that there was Elvish blood in Orcs" (sorry I don't have the actual quote at hand).

Moreover, the Mannish origin for Orcs depends upon the round-world cosmogony. If you accept the flat-world version as the "authentic" one (as CRT did), then you have little choice but to say that Orcs are corrupted Elves; they first appear long before Men awaken.

Therefore, it seems that they must posses a fea, somehow.

Agreed. Consider, for instance, their dialogue in The Hobbit.

Could the Dark Powers 'transplant' a creature's fea into a new hroa?

This is an interesting theory, and sounds plausible. A question, though: what about Treebeard's comments on trolls? He suggests that the relation between Ents and Trolls is analogous to that between Elves and Orcs. If Orcs are indeed Elvish, does that not mean that Trolls are Entish? (I realize that it is not certain that Orcs were bred from Elves, but it is at least a strong possibility). Of course, Treebeard's accounts are often vague and slightly inaccurate (as when he talks about the "great darkness" coming in and driving out the Elves).