As for tiles vs panels it depends on the overall look you want. Our bathroom was mainly tiled so the panels added a nice contrast / feature. Had we gone tiles all round it would have looked a little too tile-ey…. if that’s such a word
I followed the panel makers recommendation which was iirc a 10mm bedding of the PU colour matched sealant they provided.
I did use a sealant strip on the back of the shower tray to seal it to the wall and the panels, just as a belt and braces thing. But you can't see that, it's just behind the panel.