1.PE Pipe thermal expansion is mostly 10 times of C/S. For exact value, check with vendor. Tight clamp welded to structure sounds like an anchor to me. Do you prevent the solar effect to the pipe? if not your pipe can be heated up to 50~80℃. Either pipe or support will suffer from it.

2.Not sure what is slot piping. Are you saying that the coupling in your drawing is not fixed?

3. Guide support is possible, but stopper it's complicated. Check out the PPI(https://plasticpipe.org/)

How long is your pipe line? If line is long enough, I strongly recommend you to bury them.