I finally got her to show up at the Convergences portal. Even though i tried replaying the first two steps in chapter 8 about 3-4 times without results prior to my last post, i tried three more times today before she simply appeared at the portal. I didn't do anything different across all of the attempts, just let all the dialogue play out fully and left Mabon's room instance via the on-screen exit prompt. (Unsure if using the on-screen prompt instead of the quest pin prompt matters at all, but may be worth poking at.)