Vacformed plastic is very difficult to heat without warping, especially with pieces that have details or corners/angles, that being said I've had success with other troopers armor by using blocks of wood either side of a distorted area, gently heat the area then clamp the wood together (I use MDF as it's smooth and doesn't leave grain marks. Not something you want to attempt without a lot of practice, luckily you won't have to with the DO's allowing it.
Sounds like you have space to trim that piece off, but if you don't want to remove that piece next to the han snap you could always remove the snap, glue a piece of plastic behind the hole then fill the hole with ABS paste, sand and polish then drill a new hole.
V Tab info
With a heat gun you bend strips of ABS plastic into the shape of a V, only glue one side and the other remains free to allow movement when you bend. Just helps stop the butt plate from pushing back from the back plate.
A couple of builds with V tabs