It's good you've got some progress.
But it's going to be a long, long slog. I'd expect a repeat of this dynamic (you and LordL make a decision, MIL tries to plow over it, you freak out, MIL lies, you freak out more, FIL intervenes, MIL gives a half-donkeyed apology on facebook, LordL tries to give in to MIL because she's said sorry, you freak out more, LordL backs down again, rinse and repeat) to go on for several *years* before you come to a place where LordL isn't so much under his mother's thumb. If it goes well.
I'd very, very strongly recommend family counselling - not necessarily just couple's counselling, but meeting with someone with expertise in family dynamics and dysfunctional families. They will have seen this exact same situation hundreds of times, and can provide useful strategies for coping that will hopefully strengthen your relationship, rather than stressing it.
If you plan to have kids, I would multiply the above by a factor of 100. A MIL who doesn't recognize healthy boundaries (and will lie to your face while she tramples them) and a husband who reflexively gives in to his mother is much, much worse when kids are involved.