theque, I don't think you are alone in what you are going through. I think many people find polyamory from falling into it as opposed to deciding that it is what you want from the beginning. So like any other major change to your life, it can leave you very disoriented until you get your bearings.
I like your view that it is a continuum. Polyamorous people do tend to vary in the openness of their relationship from polyfidelity to complete openess with no rules whatsoever. Most poly people fit somewhere in between.
It sounds like you want to get over your anger and fear of Justme and your friend being together. Is that right? I had a hard time visualizing my wife with someone else. However, once I really thought about the idea of compersion (or frubble), I realized I could focus more on her happiness and try to focus less on my jealousy.
I would suggest reading some books on polyamory if you think that could hel you. Sometimes it takes listening to what others have been through to help you find the ground for your situation. If you listen to podcasts, you could listen to Polya,ory Weekly. That is a very good forum for a wide range of polyamory issues and discussions.
I hope you and JustMe can find the happiness you are looking for.