Problem was that the S'ona were dying
Probably becuase they were using crappy non-federation medical tech, just switch over to federation medical technology. It kept McCoy alive so it should keep them alive long enough to figure out how to make their own magic youth radiation.
As for the partial claim thing, thats a dispute they need to work out with their parents, since the federation is around they could possibly mediate it. I mean the Ba'ku didn't have a problem taking them back at the end, the Son'a seemed to be the only bitter ones there.
That part of what the Son'a told Daugherty was probably true: they didn't want to live in the middle of nowhere. They wanted the benefits of the radiation without having be stuck in the Briar Patch. They did, after all, develop the collector with that singular goal in mind. I don't think the Son'a would have come up with such an elaborate bit of kit (which must have taken many years to research and develop) without fully intending to just take
the radiation and leave their parents to rot.
If the Son'a were willing to return in peace to their people, I don't see the Ba'ku telling them to get bent. Indeed, as was shown in the film itself, those who wanted to come back were welcomed with open arms.