Have you tried weston-launch the right way?

That is, have weston-launch setuid root, then in a text mode VT, running
'weston-launch' without any arguments as a normal user?

Though, given your other problems, I don't expect that to help.

Also, sounds like you build weston with ./configure --with-cairo=gl or glesv2.
As GL clients do not work (weston-desktop-shell crashing - that's probably yet
another bug as the fallback fails), this causes the desktop (panel, wallpaper)
to never come up, so you are left with a black screen also for that reason.
Using the default --with-cairo=image would remove one "moving part" from the
equation.

I'd expect people to ask for a full kernel log from boot up to and including
the problem.