A very interesting essay, and a respectable attempt to find a solution to a problem about which, in the end, Tolkien himself could not make up his mind. However, I fear, the attempt you made to resolve this issue is doomed, because the conclusion to which you come contradicts the behaviour we see from the Orcs in the published LOTR-books; as did the attempt of Tolkien himself to deny that the Orcs were Incarnates.
I mean the points:
• They do not have souls.
• Their wills and actions derive from Morgoth.
• They are not directly descended from Elves and Men considered as whole, incarnate beings ie those in whom fëar and hroär are properly integrated. Instead they were derived from the physical, genetic material of Elves and Men, either from hroär from which the fëar had been driven, or from tissue samples taken from living Elves and Men.
I do not think this works. Because whatever Tolkien wrote to try and bring together his two statements that
a) evil is barren and
b) only Eru could provide souls,
the fact remains that he had portrayed Orcs in LOTR - that is, in his published work - as beings who clearly acted and thought and talked and had a will of their own, even at a time when neither Morgoth nor Sauron were avaiable (Morgoth being banned beyond the Door of the Night, Sauron being recently slain; and still, the Orcs attacked Isildur, and won, so they obviously used tome tactic to do it); and later, in the Book TTT, Gorbag and Shagrat do not act as if they have no souls or are incapable of rational thought, either; had they been directly directed by Sauron at that time, the whole mission of the Ring going to Orodruin would have been doomed. And also, the fact remains that Tolkien had stated - not only in the essays about the origins of Orcs, but also in the tale about the origin of Dwarves - that only by adoption of Eru the Dwarves could both move without Aule directing his will on them at all times, and try to escape destruction by their maker (Aule, that is). The Orcs showed a remarkable talent for self-preservation even when Sauron and Morgoth were not there to direct them.
So, I think whatever Tolkien tried to do to bring this paradoxon together, it cannot get solved, which is IMHO why he gave up on the attempt. The problem is the same all christian religion has (and after all, as we know, the professor was catholic): Why does God allow evil? I think, the only solution that is working is the one Tolkien gave in the remark that was quoted (incompletely) above of Tolkien's essay "Orcs" (HoME Vol. 10: Morgoth's Ring, Harper CollinPublishers 2002, Page 411):
"But Finrod probably went too far in his assertion that Melkor could not wholly corrupt any work of Eru, or that Eru would (necessarily) interfere to abrogate the corruption, or to end the being of His own creatures because they had been corrupted and allenm to evil. It remains therefore terrribly possible there was an elvish strain in the Orcs."
(and further with the part you quoted).
I fear, in this case Tolkien's words remain as much a contradiction as his different accounts of the history of Celeborn and Galadriel. Therefore, as interesting as the approach you give here is, I fear that in the end, the matter simply must remain without resolve.