Adding the main story characters back into the band is actually harder than it would seem. And of course once I do so, players will expect content from them - band chats, hang outs, new responses to gifts, which of course I understand. They've all changed as people by that time.
That said, I've decided to let 1 person back into the band after their route. The other 2 will not be recuitable for story reasons BUT will be around. You can "hang out" with them on the weekends and I'll try to make them present in other people's arcs. Right now, just including them into Sebastian's arc is my main goal but I will try to do more where I see fit. (I'm open to suggestions.)
If you want to just have a band with them in open mode, you can do that by starting a new band and choose not to trigger any story (true sandbox). They'll be available right away then.