I mean, you can return your planes to try and curry favor, but, well, in Osea's shoes would you rather have a bunch of "totally good planes pinky promise" or "Strider Squadron"?
Well it probably wouldn't make much difference to Osea, it might make other countries less reluctant to grant supplies to XCOM. Especially if we make it clear that anyone and everyone who stayed did so of their own personal choice.
That way, everyone is forced to publicly acknowledge that the only scenario under which men who have been temporarily seconded to XCOM would not return when ordered is if they themselves decided not to. This would hopefully make other nations far less reluctant to provide XCOM with additional men and material, confident in knowing that the only reason XCOM wouldn't return them is if those men decided to defect.
This cuts the legs out from under anyone who objects to sending additional personnel to assist XCOM on the grounds of them possibly not being returned, as doing so is an implicit admission that they do not believe their troops are loyal to them, and a direct insult to their troops.
This should prevent anyone from voicing such sentiments as criticizing the military in such a manner, especially in the middle of a war, is political suicide.
TLDR: The idea is to shift the blame from XCOM for stealing to Osea, for causing their own pilots to defect.
(Also, unilateral defections has one very public association in world memory, so, you know)
(yes, Razgriz files have not yet fully disseminated)
What if we disseminated it immediately after stating that we would not prevent anyone from returning to Osea, and that we were giving back the planes?
If Razgriz decided to stay, that might even help to get the Osean public on our side (or at least hating a slightly less).
Basically, I'm relying on the fact that Osea is a democracy, which means that PR matters as much if not more than diplomacy. If the Osean public wants to rejoin XCOM, or at the very least is hostile to the idea of the Osean government acting against XCOM, then that's something we could use to help shield us from Osean retaliation.
TLDR: Osea is a democracy, let's exploit that.