Falc - I get it. You need closure. There is no shame in wanting to see her again... if, for nothing else, simply to get an explanation. And we would probably all agree with you - you deserve an explanation.
That being said, I want to give you a few things to chew on.
Scenario 1 - You go back to see her and she doesn't bother showing up. If she's involved with someone else then she might feel that it's safer to simply do that. After all, she's irresponsible enough to just leave you with all the bills.
Scenario 2 - You go back, meet with her, and she looks at you and says, "I'm done with our marriage. It's been over for a long time but I didn't have the courage to tell you." You cry and beg and make a scene and she looks at you with contempt. You return feeling worse than when you went.
Scenario 3 - You go back and she says, "I do love you but I'm not ready yet to make a decision."
For as much as it might be important to you to get closure, any one of the three scenarios are very possible. And all three of them lead to the same end - you remaining stuck and feeling shitty about the entire event.
Now, there is a 4th scenario - the one where she looks at you and says, "I choose you." The reality is, however, that is very unlikely. Why? Because if that was her thought process, or even if she was remotely leaning that way, she would be in occasional contact.
So here's my most important comment due to the above - no matter what happens... how are YOU going to look at YOURSELF? In every one of the above scenarios you, 10 years from now, are going to bang your head against the wall for allowing yourself to be treated like this. You're going to hate yourself for having had such poor self-esteem that you allowed this woman to treat you so poorly. THAT is what reconciliation, or divorce, is going to look like. And trust me, I am living proof.
It is imperative that you turn your anger into action and take control of the situation. You do that by demanding your mom's ring back - now. You don't wait for a theoretical meeting sometime in the future. You tell your FIL to let her know that you want her ring immediately. File for divorce... it takes a pretty long time for it to go through... and have her served. Begin the process of moving on and let her know that you will NOT wait for her. You're not Plan B for anyone... so start acting like it.
Then - you detach. How? You keep yourself busy throughout the day. Go out with friends who know what you're going through and ask them, personally and intimately, to keep your mind occupied. Tell them you are having trouble being alone... can they keep you company? Take you out? Go for dinner?
Replace your happy thoughts of her with reminders of what she's doing to you. If necessary, write them down and put them in your pocket to pull out when you start feeling down. Simply begin the process of moving on with your life.
We all know how hard it is. Emotions are tough to overcome. But stay objective and do what is necessary.
I am a big proponent of giving reconciliation a chance. But you can't reconcile with someone who's not interested... and isn't even present. Move forward without her and you'll find out, very quickly, if she has any interest in coming along.