"Fatigue" is not well defined in BFRPG. Are there a specific mechanics you have seen that would work?
I still need to look into the whole Gaze Attack thing to see how I might approach it.
One thing I don't want is this sort of character getting to CONFESS every other person in an adventure... but I don't want to hamper the character from being able to adventure otherwise too much either. Constitution loss seems like a decent middle ground; a full night of rest puts the character back up to max, likely even if I change the restore rate to 1/hour of rest (assuming they are not extremely high scoring in CON but somewhere near average). The fear of losing CON, including resulting HP losses (from either loss of bonus or dropping so low as to have penalty) should keep characters limited to using the power judiciously.
Agreed on the Detect Lie skill... Wis Skill Check p.141 (with additional bonus based upon level, tbd)
I think a flat +2 on saves vs. charms (or mind control/reading, or similar abilities) will suffice for protection against the same abilities used on them.
I would go "Cleric Sub-Class", but I think I already warmed up to the idea that there are other 'sorts' of Confessors than just the style of Kahlan. I think Q-class will work better for that, especially since I utilize other Quasi-classes in our campaign already.