Re: Band members that slow progress in Band development.
I think we've all been there, and the only thing I can suggest is as point by point discussion of what's eating you about their playing. Pick the one problem that bugs you the most (just pick one per rehearsal) and as lighthearted as possible, tell the person that it's making a problem for YOU. (not in so many words though, diplomacy is an art.) Speak in terms of yourself, for instance, you could say "hey everytime we go into this section and the bass notes run up the neck, I feel kind of unsupported, could you do me a solid and stay down low, it would make me feel less exposed" It's important you offer them a suggestion about what you want them to do. Some people don't like direction, some people like the attention that their playing is getting and are more than happy to oblige. This will show what kind of person you're dealing with. If you have the kind of members that are happy to oblige, great. If not...?
Diplomacy: The art of telling someone to go to hell and actually make them look forward to the trip ha ha
Good luck with the singer ha ha