On regular ollie impossibles my upper body just faces the direction I'm going the whole time (maybe at a slight angle, but that's how it feels like when I do them). The trick has literally no influence on my shoulders at all and it's really not supposed to have any on yours, it really doesn't work like a 360 flip for instance where more factors are at play, and your upper body plays some kind of role in the control. On impossibles if you basically just stand straight (don't lean over the nose), just look past your nose and try to imagine scraping your tail through that zone it shouldn't really matter what you do with your shoulders, just keep them in place.
On switch ones I do have a more typical switch 360 flip kind of shoulder placement but I think that's because I learned nollie (front foot) impossibles first so I was always used to that kind of positioning and momentum. As long as you only let your lower body do the work and don't twitch out of the way, you should be fine.
Basically you just need to forget everything you know about shove-it based tricks and flip tricks for proper impossibles as they rely on a completely different technique and shape, I would say the trick functions more like a catapult (best analogy I could improvise here). The technique is similar to pressure flips indeed but in reality that doesn't necessarily have to translate over to your foot positioning and perception of the trick because you don't actually want the board to start flipping like a pressure flip, you just want to invert it upside down over your back foot and then lift that back foot up. There's some big toe action, but it doesn't have to be some funky mathematics over specific pressure points on the board, my popping foot goes across and covers my whole tail (or nose) regardless of my style in each stance. The maneuver itself is actually very rudimentary and involves less of your body than you think, in a way it's a really lazy trick (on flat and banks at least). In general the people who struggle with it are really fighting against their own understanding (or lack thereof) of the trick because it's different from the 'modern' flip tricks most are used to, but the less they overthink it the better.
As much as shoulder positioning shouldn't affect the trick too much, you do have to keep them in place though (to avoid parasite movement). But besides that you don't really need to think about them at all and should just sit over that back leg then release the pressure all the while keeping in your line.
Once you've got them down, you can actually use how minimal the shoulder movement needs to be on the basic variation to your advantage for impossible reverts or body varials both ways (the frontside body varial ones are super gross, but impossible reverts can look sick). Easier than it sounds and getting impossible reverts from miscalculating your weight distribution or upper body reflexes on impossibles is actually very natural. Better looking and feeling than shitty 360 flip reverts for sure because then the revert just flows out of the first trick instead of breaking it (if that makes sense).
If none of this works, make sure to try a few fakie ones. You'll surprise yourself with how natural the motion is with the fakie momentum.