Ugh. This is what, my fourth play through? I don't know how I went so long without realizing this, but apparently if you "indulge" in any other romantic interest, including Isabela, you forever close off the romance with Fenris. Now I'm in Act 3 with every LI closed off to me! This is so disappointing!
Ah well. I have another character who I put on the shelf who was romancing Fenris. I think I'll pull him back out, because really, all of this was done for the end result, and I don't want to complete the first two acts AGAIN, right after finishing them.