I did play the other quest first.. I saved the game before the choice so I could experience playing both quests. I wanted to play the storm the castle part so the city would get marked on my map cus I had not been there yet. oh well.. after running to it a few times with Taranor i can reload to a time after he gives me the castle, and I think I can find it again.
Edit: I went back and tried it several more times. we are at Cahbaet not Erothin, we had ridden from the Monastery ( I was mistaken about the location) ..
Callisto is there, he's behind me away from the party thats talking. When we leave for Castle of Darlan he rides his horse. a few times i remember seeing this horse stuck up in the air between Cahbaet and Darlan but i didn't releaze it was Callisto. The times the horse was floating in the air, it was motionless, so he would have been stuck.
After reloading a bunch of times, I finally do get Callisto to join us runners with his horse. i had to watch him carefully to make sure he makes it to Darlan with us. There is a spot where we enter the woods and he gets off his horse to fight. He does not get back on his horse after that. This time he made it to Darlan with us and opens the gate.