You're actually not going to have TOO much of a tough time, I think. Somebody please correct me if I'm wrong, before I continue playing and find out one of the other quests has no other option.
After playing through the initial meetup with Dorn, I slaughered everyone at the Wedding, which obviously called my Inquisitor to revert to a Fallen Paladin. I don't want that! what can I do? let's replay it... okay, this time, I talk the wedding guests to kill Colin (the Groom) and convince Dorn to let all the other people leave, so I end up killing no one- I keep my Paladin status. Excellent.
I don't remember if any of Dorn's other quests involve an ultimatum of "do this and become fallen paladin or lose Dorn from the party". If anyone knows, please share here.
As for "why would you be a paladin with Dorn in the party?! Oh, my bleeding roleplayer's heart!", because I want to use Carsomyr, but I still want to go to Lunia as well as doing all of Dorn's other quests (which means no just waiting until ToB to use Dorn).