BillJ, I think Klingon ownership / transfer of ownership of the briar patch in general, does not change the relevent fact of the Baku's claim or their long history of ownership.
Obviously the Klingons did not kick them out, and may not have even knew they existed, otherwise, their world would look very, very different.
The Federation's highest laws, and this dates back to Kirk in TOS at least, states that "your world is yours".
The Federation is an alliance for the mutual benefit of member worlds, much more like the United Nations than the Roman Empire.
even if the planet was definitively shown to be the Baku's, it doesn't really help their situation. It wouldn't prevent the Federation from negotiating with them for removal, and it certainly wouldn't obligate the Federation to DEFEND them from pretty much EVERY major power that's going to try to take it from them after the events of STIX.
What's to stop the Federation Council from saying "yeah it's yours, good luck defending it, tiny village of pacifist Luddites!"
One could theoretically invoke the Prime Directive in terms of whether or not to mount a defense, but the fact is that the Federation were partially and directly responsible, and the Baku were advanced enough that the Prime Directive was not applicable.
Morally and Ethically, Picard turned on a Federation that had turned it's back on its own core principals.
The Federation provides help and assistance when necessary, part of the mission profile of a Federation Starship.
nope, you can't have it both ways, as the movie tries to do. Picard's involvement could only be even REMOTELY justified insofar as the Federation was already involved. Once they all left at the end of the movie, there was NO reason to continue to defend them, especially as the Baku were contributing nothing to the Federation at large.
pick an option: either it's in Federation territory and they can legally and ethically remove the Baku or it's not and the Federation could have had the Son'a come in and settle their dispute with the Baku by force.
(this is usually the part in the discussion where the anti-removal crowd starts moving the goalposts and insisting that the Federation has a moral OBLIGATION to defend the Baku.)