That is a very tough situation.
With live sound the practical goal is that you may make a good sounding band sound it's best.
Since you mention compression on the vocals I wonder how comfortable you are ringing out a room and the monitors with an EQ. If you are having feedback issues I'd reserve the compression until you don't have the feedback. If you ring out the room ands stage with EQ you should be able to minimize feedback and then add compression to the mains while using as little as possible in the wedges. (in ears; you can use all you want).
If you are using wedges you want to keep the vocals as clear as possible rather than thick and meaty because it hurts the FOH mix.
I like to ring out the system and the wedges and make the sound level that comes out of the floor monitors capable of being painfully loud... that way when someone says the monitors aren't loud enough I can make it really clear that no one really want's the monitors to be louder... they just think they do... So I let them hear them that loud... and then we can introduce seemingly reasonable ideas like turning down some guitar amps etc.
If the band hasn't worked out how to manage their stage levels a live sound mixer can't help them very much.
A lot of not in-it-for-the-money bands are lacking a leader and when it's like that it just is what it is... no one is really authorized to tell another to turn it down without creating resentment. I totally get how much fun it is to play with friends and avoid disagreement... but if that's what it is... that's what it is.
Great bands just mix them selves on stage. They play with each other. They don't need a leader... the music is the leader.
If a singer wants to share a stage with loud rockers then she has got to blast it into the mic... that's the only way to make feed back and bleed a non issue.
If that's not a natural thing that she enjoys then there is a mis match with her and the band and the best you can do is try to keep it fun and wonder how long it will last.
Stuff like that.
best regards,
mike