I'll be honest: I am seeing a lot of problems reading your post, and they don't have anything to do with the singer or the guitar player.
You say that you are part of a "pro core" of the band. Is that the shared perspective between the three of you? If you guys are truly "pro", then I don't understand why you hired sub par players. Or do you just like to see yourself as part of the core and it was really the others who took the decision? And if you so meticulously divide players in "pro" and "not pro", how sure are you that the others don't see the guitar player and singer as "pro"? How sure are you that the others really see you in that category? How do you, in your view, distinguish yourself from them, and may there be anything that these guys bring to the table that is really valuable?
I am asking all these questions because, when you go talk to the drummer and keys player about others not being up to snuff, you wouldn't be the first musician ever to be in for a very uncomfortable surprise.
And then you need to look at replacements. You say you have one gig per month - that is not impressive at all if you want to attract true professionals (I'm assuming the three rehearsals you mentioned were a one time thing before you got the gig and you don't rehearse three times every month, because that would be... not so good). What more can you offer those players? Have you discussed with the others the requirements of playing in your band? Is it just skill or do personalities play a part? Are you okay with your band members playing in other projects as well and maybe not prioritizing your project (because right now, I don't see how a professional player would prioritize a one-gig-a-month start up)? And, most of all: if you hire because of skill alone, what do you do when a better bass player becomes available?
These questions may sound harsh, but I think you better go into that discussion prepared for all eventualities.