I read your post a few times and it looks like you’re trying to solve problem you dont even have.
First off, step away from the propane torch. If you anneal the case head area you’re either going to be in a world of hurt or have a cool story to tell. Don’t do it. Dont even think about it.
The best way to handle belted cases is to pretend it doesn’t even have one. Shoot your factory ammo, or loads put up in new cases. For the intial setting of the FL die screw it in until it touches the shell-holder, then back it off a half to a full turn. That will eliminate touching the shoulder at all. Size a few and test chamber them. If they chamber without trouble load them up. Continue loading and shooting until such a time that the cases dont chamber easily. That day might never come, depending on how hot you load. If you get the urge to mess with the die, fight it off. Go weight sort some primers, or distract yourself with some other problem you don’t have. Just leave it alone.
Lets pretend that after nth firing and resizing that you detect a little more effort than you want dropping the bolt handle. Now start screwing the die in with tiny adjustments until the bolt closes easily or with just a hint of resistance. Your choice, its your gun. You can take the striker assembly out for more feel if you want, but I seldom bother. Lock the ring down and you’re done.
Thats basically the way I for all bottle neck cartridges that i dont have Competition shellholders for. The minimally sized case will keep the casehead tight enough against the boltface that the case has practically nowhere to stretch.