wrong order of operations IMHO.I would do in this order:
1. ) Look for her replacement, privately and find a potential one or a few.
2.) Meet with all the other members of the band minus your diva member, and express to them what you say here, and propose the solution of replacing the diva and have the info of replacements ready for their consideration.
3.) Get universal consensus on replacing diva, even if there is or can be no agreement yet on WHO that replacement will be.
4.) Fire the diva.
5.) Get on with your band and life and don't let this diva paralyze everything. Go around your diva in a workplace agency situation and if that means talk to other bands or other people in them to get a direct-dial and direct contact of the desired agent, DO THAT.
Good luck with it.
If you start with 1 then the next step should be 4 in order to eliminate the possibility of her getting further unhinged by thinking you were working behind her back. I can almost guarantee this WILL come up and shell bad mouth you guys for ever to everbody she can.
Stand up straight, tell the truth and take the hit.