Yes, if you fail he despawns. I found out when trying to solo him once with my DK when I got distracted.

Also, as for the statement about hoofing it. You can summon people using the stone in the middle of the mountain. But, yeah what I did was just run all 10 of my guys to the entrance of BRD. I did this last week in anticipation of the event. Then just zone in, kill the dwarves on the right, then click the controls. This will take you directly to Coren's room. So it's not really difficult.