Hyperion's H2 port uses cdda.library to play CDDA on old style analogue CD-ROMs. There is a MorphOS replacement for this library - check on Aminet.
The smacker codec built into the game is broken so I don't think you'll ever be able to get videos to play on MorphOS.
Heretic 2 doesn't allow you to define custom screenmodes. The best you can do, possibly, is make a screenmode of your own at 960V so that it fills the whole screen properly.
Time has not been kind on what was one of Hyperion's less broken ports...
Thanks for the tip about the replacement library. That's a true shame about the incompatibilities.