LotR -- Book 3 - Chapter 03 - The Uruk-Hai

[Rimbaud]

If one were really to take the argument of repenting orcs seriously, you would then have to think about what would happen to such an orc. It seems unlikely that their immediate society would tolerate such reckless good behaviour and such a person would doubtless, and probably literally, be out on an ear in no time.

The same is roughly applicable in reverse to the Elves.

Thus, it is unlikely that such characters would turn up at any major events, being dead, or in hiding/exile. This may be glib, but it is less awkward than writing in the doubting orc who was somehow embraced by his bloodthirsty brethren.