KWin doesn't require XWayland, it provides XWayland. Wayland is actually just a protocol, which must be implemented in a compositor. It is different from Xorg where the X server and the compositor where two separate things. SailfishOS has its own Wayland compositor based on QtWayland which doesn't provide XWayland. Plasma Mobile uses KWin, which support both Wayland applications and X applications through XWayland. Porting Plasma mobile to Mer should be quite easy. They said their first builts were on Mer, but that they moved to Ubuntu because several people in their team were already familiar with Ubuntu's build system for working on Kubuntu.