Question said:
Well we got the message that that part of the puzzle appears to be solved, but thats it.
The real puzzle is: Why are you guys still playing this?
Best I can do:
"For they sleep not,"
The corpse should be sleeping eternally, but he is not.
"except they have done mischief;"
The corpse was probably a bad guy who did good (considering the place you are in) as opposed to a good guy who did bad.
"and their sleep is taken away,"
The corpse is aware, hence, his eternal sleep was taken away.
"unless they cause some to fall"
The corpse has to cause some to fall (note: some, not one). It is unlikely that it refers to the corpse physically falling, rather the corpse has to cause several Good PCs to do Evil acts. It could be claimed that this has already happened with your PC changing and attack the party, but there could be more to it, but that is only one PC, not several.
"They" in the riddle appears to refer to the corpse over and over again.
I suggest that you guys burn the corpse or hack at it or some such (or even sprinkle Holy Water on it or do Turn Undead on it, i.e. positive energy). That could be construed as an evil act since the guy is eternally aware. Worse case, another PC will change and at least the "some" part of the riddle might be satisfied.
Outside of that, I got nothing. Silent Bob???