So if he glues the wall on one side to the wall, the floor, and the ceiling with the special adhesive from the board company, it will hold, I think (but I have no experience with that). The corners still with sealing tape and the joints with silicone.
The shower screen is anchored (on one side) IN the room wall, with metal pins that reach into the joint between the two Fermacell (or similar) boards. On the "front side," where the shower screen ends in the room, there is a stainless steel profile (50 x 50mm) into which the boards are inserted. The hollow space in the pipe is filled with cement adhesive. The pipe profile is firmly screwed to the floor and ceiling.
Hmm, I would definitely consider sealing there. I don’t know if sealing tape would hold on "Irgendsonem" steel pipe. Possibly tub sealing tape for steel tubs... But mechanically, that should fit.
I don't care how you want to anchor it with pins in the wall or otherwise. For the shower an absolute no-go for me. Just to gain 2cm, such a makeshift job?
50mm drywall profiles. Everything else is botched work in the bathroom.