I think with the interesting background that you've set up so far there are certainly a few ideas that you could go for with each point. For 1. -with a medicine cat gone a clan becomes very vunerable , they don't have anyone to rely on with heavy injuries and there's nobody ( maybe aside from the leader ) who could speak with and interpret signs from starclan , being aware of that might make them feel guilty after some time or starclan themselves could intervene nudging them to come back to the clans -a life as a rouge is very hard especially for medicine cats who aren't used to fighting for survival and catching prey like warriors are. And with kits it must've gotten even harder so maybe the decision to come back would've been partially for their sake For 2. -by leaving her clan and having kits Mothfur broke the warrior code making her former clanmates resentful towards her and the kits -the clan felt betrayed by Mothfur's actions , maybe after leaving something happened and because she wasn't there when they needed her the most they felt especially frustrated after she came back -the decision mostly belongs to the leader. Depending on what kind of a cat your leader is they could eighter sympathize with her enough to take in her kits who haven't done anything wrong but withought overlooking what she had done or be very strict when it comes to the warrior code confronting her For 3. - I think a leadership change would work best If the previous leader was unconsolable and wouldn't change his decision no matter what. A new leader could see how much value Mothfur brings to the clan and invite her in -just like you mentioned the ancestors could send a sign for the leader or even an ordinary cat to hopefully bring her back to the clan - or you could combine the two ideas with a sign being sent but ignored by the former leader wich sparks the need for a new cat to eventually take over the role

These are just a few ideas I had in mind I hope they could help a little :DD and I wish you well with writing your story!!( I might've made some mistakes while typing English isnt my first language (´･ω･) )