It sounds like the disorganisation of the situation is getting to you more than "stage fright" by the sound of it.
Talk to the guitarist & drummer's Mum (the "Mum-ager"?), try and get more info, especially regarding set length, ask her a bunch of questions to ask the promoters (though be careful or you'll end up "volunteering" your backline).
Instead of rehearsing one week, maybe take the time to have a good talk with the guitarist & drummer about the band, and find out from them where & what they want to do. It sounds like you need to pre-arrange a set list a few weeks before a gig, and rehearse the set prior to the gig.
Mention that this "play it by ear" attitude is making you uneasy, but also mention that you really enjoy being a part of the band and that you enjoy your musical interaction with them.
Remember, like all relationships, good communication is key.