Ok I finally got X-Mas Disaster running in ClassicWB/A1200 (020 FPU 4/16MB) WinUAE 1.6.1.
This game is simply awesome, theres a few bugs (as you mentioned) like random flashes (more noticable in fullscreen with scanlines)
but bugs are to be expected on a game that only took two weeks to make!
I love the music, feels very Amiga and gamplay is excellent.
Things I think could be added later to make this game complete are extra weapons and final bosses
Overall I would give this game 8.5/10
It could be said that this game could have been an official release back in the 199X's
Excellent Effort, Merry Christmas