Kerran can be reinstated in the templars under supervision if you have Merrill in your group to determine if he's possessed, choose to.. not sure if it was just let him go, it's not to turn him over.. and then you go to the opening area and vouch for him (I think you still need Merrill in the party, can't remember). As you first load into the area, on your right by the weapon and armor shop, you'll see Kerran, Cullen I think it was, and maybe an onlooker. They'll be talking a little bit. Talk to Cullen or whoever the other templar is to vouch for Kerran's clean bill of health.
Rule of thumb in DA2, there are a lot of choices that only show up if your character has a certain dominant tone or specific companions with you.
edit: Funny. My first two Wardens started out pretty promiscuous, so my fCE slept with Cammen, then my mHN slept with Geyna.
It wasn't as much fun getting them together, lol, but you could do both with high enough persuasion.
I went back and replayed the entire Deep Roads on my first DA2 playthrough to save Bethany and make her a warden. I really didn't like taking Anders on that trip because my goal was to take Bethany and Merrill (Hawke family present and future
), and Varric was required. Wish we could have taken three others and had Varric as a come along there. It's one of the best moments in the game though, I guess, but it was frustrating.