Well, Sassy, thanks for joining us. This is exactly why our Forum was created. Not for bashing but for mutual understanding, support and brainstorming.
I would say you have a bear by the tail. It doesn't sound to me like your FMIL has passed the "letting go" test.
We moms must necessarily start out obsessing about our kids. If we don't, they won't survive. We have to monitor them 24/7 and the responsibility is awesome. As they grow, our duties change but they are still constant. As a result, it's almost natural to establish an identity in the mom role that may develop a life of its own, ego-wise. (This can't help but be intensified for a single parent, by the way.) Then there comes a time when we have to start shifting responsibility, a little at a time, to the emerging adult and for many moms that is very difficult because too much too soon, is as bad as not enough, too late. Tricky stuff.
If the transition doesn't take place, then we have an over-involved parent who may see a "we" still existing as marriage approaches, along with vacations and the possibility of grand children. If that isn't acceptable, who do you think is going to be blamed? As in, everything was fine "before!" (However, if you ask your guy, everything was probably not fine and it's probably been a struggle for him for some time.)
I have seen this kind of super-attachment in friends of mine, who insist on knowing every detail of their adult children's lives and who want to participate in absolutely everything...even seeing it as their "right." Some of us have the opposite affliction, of course. Case in point, me. I can remember when my youngest son boarded the bus to travel to another state to enter a protestant seminary to become a youth pastor. There I stood at the bus station with tears streaming down my face, sure that his leaving would kill me...while at the same time, I knew that if the bus stopped and he got back off, I would Kill him!
And do you know what? That son now often introduces me by saying, "This my oldest friend...my mom." (He design and created this Web-forum for us, by the way.)
Occasionally, over-involvement can be readily accepted and highly valued. Some married, adult children thrive on it. I have a close friend whose two daughters wouldn't file their nails without calling her up and talking it over with her first. They both call many times every day. Give me a break, where's the chance to learn to make wise decisions or to face the consequences when that doesn't happen? I think a healthy separation the stuff independence and maturity are made of.
Think about this...what if your FMIL got a DIL that wasn't like you and wanted the guy all to herself/? What if she either attacked her MIL directly or went after her, under the guise of friendship, and subtly manipulated her husband into participating in a full-blown character assassination, as happened to Prissy? Your FMIL probably doesn't know it, but things could be a whole lot worse!
I see this as your to-be husband's problem. It's time that you make it clear that his "if I ignore it, if will go away" attitude isn't going to cut it. He's the one in a relationship with his mom and he's the one who is going to have grow through it with her, no matter how painful. Denial of the seriousness of the situation is only going to set you up. That in turn will complicate your marriage in ways you can't even imagine.
Good for you for addressing this in advance and thank you again for coming on board. Blessings, Luise