@chris10. I tried adding several epithets in an ongoing campaign and everything works without any problem. So I guess adding traits (as long as it's at the bottom of the list) is OK, but maybe modifying them in game could indeed cause problem.
@Gaius
First of all take a look at this: http://forums.totalwar.org/vb/showth...omats-assasins
Put your new traits exactly where he says. Don't forget the export_VnVs.txt to add matching description of the trait.
Here is an example I used,
in export_descr_character_traits:
And in export_VnVs:
I didn't mess with the triggers (I just add the traits via console when appropriate), so you will need to ask somebody else if it's still a problem