I'd try lightly (and I mean lightly) heating up those problem areas with a hair dryer on low and see if you can tease out the compressed areas with your finger. It shouldn't be any different to creases in other Superhero suits which typically spring back or "self heal" with a bit of heat.
I was considering going the "pin the in back" route, as my preferred pose would be him hovering slightly higher than the others. By using a pin into the back of the body there is no clamp needed. But I need to investigate how to get the pin through with minimal damage to the suit.
I'm sure others have done this, and I'm surprised Hot Toys haven't adopted this as any small hole in the suit is covered by the cape.
Open to more info from anyone who has tried this.