And seeing as the Ba'ku claim to the planet predates the existence of the federation by a few DECADES its kind of hard to say its a federation planet. Especially since the Ba'ku were unaware of the federation's existence until they were trying to kidnap them.
That depends. If the area was controlled by another power prior to the Ba'ku landing there and then was ceded to the Federation via treaty or war. Then the Federation would have a viable claim of ownership.
Even then, the S'ona would have a claim of ownership just as strong as the Ba'ku. Just because they were forced to leave, doesn't mean that their ownership claim would be invalid.