Fight and Assault should do double the damage, if nothing else. Dealing 5 damage is a joke, barely a flesh wound, while dealing 10 damage would be decent, but it costs 2 AP, 2 freakin' AP. Fight should do 10-15 and Assault should do 15-20 damage. Don't forget that if you attempt attack someone, you need to find the tile they are going to step on, which is already hard in itself.
I'm against Fight and Assault being able to stun. Stun should be the extra bonus only achievable by reflecting back an attack with Defend/Cautious. They shouldn't do double damage though, they should only reflect 100%, not 200%.
And here is another suggestion. Defend, Fight and Explore should both cost 0 AP, and Cautious, Assault and Investigate should both cost 1 AP. 2 AP is just not worth it, unless it hits, but that is rare. Though the extra compensation (Invigorate) was a nice touch with them, so they could even stay however they were.