That seems tricky in diagnosis, all I could think of would trying to exclude possible reasons one by one...
Regarding reinstalling MPlayer-nodeps.tcz:
1. Compare md5sum of your local extension with the one on repo
2. Boot with 'norestore' (and preferably with 'base') as there might be prior config files in /home (or elsewhere in backed up files).
I would try to play some other (arbitrarily) .ogv (theora) files and see what happens.
You could use 'mplayer -v' to possibly get some more precise output, but that can give so much output that the task may start like searching for a needle in a haystack.
Not sure if the 'illegal instruction' in vlc could be related with your error output on mplayer, there is a chance that might be coincidental
By my estimation, to debug vlc would probably be more effort than to debug MPlayer-nodeps, due to the difference in complexity and dependencies (not saying you didn't do well to try another player in order to compare).