1. Yes, but there needs to be a consensus on how this will work, for example - first come, first served as far as gigs go. If I’ve agreed to do a gig, that gets priority. It goes in other band diaries as an unavailable date for me. If a ‘better’ opportunity comes along, I won’t blow out the existing one. I may try to negotiate an arrangement so that I can do the second gig, but I’ll never leave a band in the brown stuff.
I’m in the slightly unusual position of being an actual member of two bands (one a startup not yet gigging, the other gigging infrequently) and depping with at least five others. I mentioned diaries - some of those bands use the Teamup app, which is free and pretty simple to use. It saves a lot of to-and-fro messaging about who is available for what dates.
2. It sounds like you’re the bandleader here. I’ve done that twice - never again! I don’t think I can help with this one, other than to ask: If they lack motivation to play with your band, they probably shouldn’t be doing it!