Toilet front mounting element before 1m knee wall on underfloor heating?

maybe generously open the wall behind the toilet

You mean basically remove the knee wall, and instead place the pre-wall element there?
Then I would also have to close off the space behind the pre-wall element again. Otherwise, the pre-wall element would stand unprotected under the roof truss, if a rodent decides to cause trouble there, cheers.

That would be too much effort for me. And I already have 20 cm of space for the pre-wall element. In the end, I’m just swapping the old cistern for a pre-wall element. The toilet therefore stays roughly in the same place.

But back to the floor fastening: would you first apply leveling compound, and then place the feet of the pre-wall element on it (construction/assembly adhesive because of underfloor heating)? Or rather smooth the screed at the spot and place it there, then level afterwards?

Or do I have no choice and should try to expose the heating pipes at that spot in the screed in order to be able to anchor the feet into the floor between the pipes?