Sounds like you have a good start going there. Great news. We don't get that much around here
I think if I were you I would leave them alone for now and see where this takes them. This is their relationship after all, not yours. Its their choice what they do with it and how invested they become. Really you are the one that has to figure out what you will do with your time when she is gone out and what plans you have for your future in this... that is enough no?
As to him visiting... bring it. Strong metamour relationships mean a strong back bone in a dynamic such as yours. You might have to look "metamour" up in the defintions sticky
Do some looking around here. do a tag search for "vee" and "secondaries" to get a better idea of what might be going on for him... see where that takes you. There have been some good blogs suggested too that would be worth reading. Mine is about me as a hinge with my two live in guys mostly... one of which is my husband. We started out much like you did, almost three years ago. You are in good company here. Enjoy the read