Im no expert by any means but I hate to see someone about to give up. You say the idle screw won't adjust the idle, is your throttle linkage in a bind? The last time I needed to lower my idle, I didn't have any slack in the linkage so when I made the adjustment on the carb the linkage wasn't letting the slide actually close further and I didn't see any difference.
I had a traxxas with a 2.5 when I was young and we had so much trouble tuning it. After some research, I blame it on the traxxas fuel. I don't see any other reason it should have been so inconsistent. I've found quite a few people on the internet saying it makes motors harder to tune and run hotter compared to other fuels.
I've been using VP fuel in my most recent car and its been great. I spend a couple of minutes getting the tune close when I first start my day and usually never touch it again unless the temp swings a lot. I started out running their high oil 20% and ended up switching to their 30% lower oil content fuel. It ran great on both, I had no problems tuning for the switch either.