Simple question that I couldn't find here already:

What are the requirements to have Aerie as a third member of the relationship vs. just a friend to Charname and Imoen? Are they based on Aerie's romance variables, or based on talks with Imoen?

I don't know it goes to a 3-way if that's what you mean. Youd have to search the code for where it sets aerie to 3 or 4 when she asks about the relationship, and not have the value changed.