I have the exact the same problem after doing everything in the exact order you described.
The bodies of all three bosses have now disappeared along with their quest markers, but Halsin still won't acknowledge they're dead, although my party has killed literally everything in, around and under the goblin camp. And Zevlor now has a new quest marker about the goblin leaders being defeated, but he won't talk to me about it.
And yes, it's also an honour mode playthrough, so no reloads.
Ah well, no tiefling party for me this playthrough, I guess.

Edit: I thought that killing Halsin might at least progress the quest with Zevlor. It doesn't, lol