All powerlifters train back heavy and they do have injuries but it's allmost always due to rotators. I don't consider protracted shoulder girdle that common, but it can be apply to them sure. It could be different in other movement but in bench press I doubt it really effects anything. Once your scapula is locked, it's pritty much only your rotators in use. I couldn't find any releated info of this concerning bench press.
I think your view of back training in powerlifting is too reductionist. I'll agree that the rotator cuff bears the brunt of it but in practical terms it's not like the rotators work in isolation, they are working as part of the entire upper back musculature.
You're right about hamstring in deadlift. They have to be movers. I'm doing sumo so it's very minimal there. I actually do heavy hamstring workout 2 days before deadlift/squat and it doesn't effect me. Maybe conventional could be different story but I haven't done it in long time because sumo fits me better result wise. For squats I don't feel them eather in normal stance. exrx counts it in squat as Dynamic Stabilizer but it can also be target muscle. I actually don't know.
I agree that you don't really "feel" the hamstrings during squats, but they are contracting very hard indeed. If I remember rightly, the co-contraction of the hamstrings and quads in hip extension is known as Lombard's Paradox. I'm too lazy to google it and check but I think that's right.
Yeah Louie Simmons's stuff is all about geared lifters, but he makes some very cool points about assistance exercises which is why I mentioned him. I just find it very strange that, as a powerlifter yourself, you don't see the value in assistance exercises.