Yes, leaving is still an option. However, the band leader has been handling things much better after I sent my email that I mentioned in the original post. He's being respectful and our last practice (last Wednesday) went well. And, I'm treating this as a temporary situation. I still have an ad up on Bandmix and I'm continuing to look for other opportunities. For now, this is a good situation to recover my chops. If the poor behavior hadn't changed then I would still be considering leaving more immediately. But they are making concessions at this point. It may just be because they need a guitarist for the gig so they don't want me to fly the coup. Or maybe they talked things over and why I was upset and trying to make an honest change. Either option is okay with me. I understood the circumstance that I was a temporary replacement before I agreed to do the gig. Pro guitarists get hired for this sorts of gigs all the time. I just have to make piece that I'm not a full member of the band, so I don't have a voice in song selection or for refining arrangements. I make suggestions, but I have no expectations that anyone will act on them.
Where I got bent out of shape was when it was suggested, right in front of me, that the band might curtail my opportunity to practice so soon before a gig. Suggesting that I learn 40 songs with unique arrangements and restricting my practice time with the band, that's something that pissed me off. It would have been completely different if, instead of talking as if I wasn't in the room, someone had asked "We're thinking of having you come to limited practices because the practice room is too small for the entire band roster. How do you feel about that?" Then I could have said something like, "I understand that this is an important gig for your band. Reducing the number of practices that I participate in so soon before a major performance will compromise my ability to give you my best performance. I'm not comfortable with this and I hope that you'll reconsider taking this action."
What was hard was being in the room while they talked without letting me voice my opinion - or at least ignoring me as I spoke. Then, having the band leader pile on by micromanaging my playing and giving me inconsistent instructions, that was just the coup de gras. I used to read Tommy Tedescos column in Guitar Player magazine where he would discuss studio sessions and how he would deal with unreasonable producers. As a studio musician he frequently had to deal with difficult personalities making unreasonable decisions. I still remember a story that he shared where he was at a session and the producer asked him to use a balalaika instead of a nylon acoustic. Tommy Tedesco had a half wall right in front of him, between him and the control room where the producer was sitting, so he bent down and pretended to change instruments. He played the next take with same instrument he was originally using and the producer said something "Oh yeah, that's much better."
I am far, far from Tommy Tedesco's skill level. I'm a weekend warrior whose chops have gone to seed after having a fretting hand injury for the last decade. Regardless, I want to be as professional as I'm able. So I'll put up with some crap if I believe it benefits me in the long term. In this case, having a band context to improve my chops and the opportunity to get current recordings of a performance makes it worth dealing with some amount of shit. There is a line where I'll go "to hell with it." But, for the last week, things seem to be moving further from the line, so I'm sticking with it unless there is some drastic change for the worse.