I'm going to be blunt and ask why you think it's a good idea to marry into this crazy? I mean, it's like an onion with multiple layers of deal breaking problems and a whole row of red flags flapping in the breeze. Relationships aren't supposed to be so hard, so where are your standards?

You should require your BF to get his issues sorted before you marry him. That means he gets legal and physical custody of his children, gets a c.s. order for BM (because it's not your responsibility to raise or support his kids), and puts his hostile, toxic mother in her place. A wedding symbolizes a beautiful beginning, but this guy has a ton of unresolved carp that's impacting you. Don't be okay with that.

Your BF's mother sounds mental. You shouldn't have to even speak with her, and your BF should protect you from her...which calls for uber strong boundaries...which your BF can't implement until he gets custody sorted...

Please, please don't marry this guy unless and until he's done what needs doing. He should want to do whatever it takes to lay a foundation for a successful marrange to you.

I will echo the sentiment that you need to work with your FDH prior to the marriage to figure out how to establish some boundaries with his MIL. My advice is couples counseling, because this will drive you crazy. 

Right now, I deal with MIL by being nice to her face and avoiding excessive contact with her. DH - who used to just give in to his mother like your FDH - just ignores her almost entirely. DH and his siblings all have a dysfunctional relationship with overbearing MIL. They are happy to take her up on her boundary-busting behavior when it suits them. For example, before he met me, DH relied a lot on his mother to help with the kids. He always assumed she would help, which she did, but then she would act like she was a parent and entitled to the same rights as a parent in terms of seeing SSs, deciding what activities they should participate in (she would sign them up and then tell DH how much he needed to pay), deciding to schedule family gatherings when it wasn't DH's weekend and then just assuming DH would make offer the necessary favors to BM in order to make it happen. She also started to tell me what I should be doing with the kids and how much money DH and I should spend on the kids. It's caused major problems for DH and I...problems that have driven a huge wedge between me and DH because I resent his family who is unwelcoming and overbearing and that makes him feel bad, because they are his family and he is either unable or unwilling to see their dysfunction. His brother and sister also love to take MIL up on her offers of help when it suits them, but then get angry at her when she oversteps. It's a whole web of dysfunction that I was not prepared for and now I'm in the middle. You need to have a long, difficult discussion with your FDH about the role he plays in creating this dynamic with his mother and develop clear, consistent strategies to remove that dynamic from your future life. Otherwise, MIL won't change and she will also be trying to interlope on your life, not just on the life of your Skids. She sees a weakness in the fact that your FDH won't stand up to her and that you, as the soon-to-be-SM don't yet have the social capital to put her in her place, but she will try to do the same thing once you are married. 

were the youngest kids given over to your 2b-MIL instead of to Dad is the first place? Unusual. Sure temporary, but seven years is a long temporary. 

I'm going to bet 2b-MIL has done a huge share of raising these kids in those seven years. Then you walk in 18months ago and 8 months ago move in with Dad and kids are calling you Mom. I'm not saying that's necessarily a 'bad' thing, but these kids should have never been left to Grandma in the first place. I know, can't undo what's in the past, but it seems Dad is still letting his mother be 'mother' or at least until you squeal and stomp your foot. 

You need to stop acting like this woman is the big bad enemy. Your 2bDH put her in the spot she's in and it's only natural the woman is putting all the blame on you. After-all, her son let her raise the kids and rule the roost.  This situation pops up here every now and then. Where the biological father has let his mother have way too much power and control of his kids... right up to the point where the new Gf/SO walks in and objects. Then the new SO/GF wonders why the Grandma has troubles adjusting. *sigh* 

It shouldn't be you pushing to stop the 'temporary' guardianship, you shouldn't have to 'push' him on it at all. It should have been done long ago and it should have happened long before your arrival. Can I ask why your 2bDH never had a relationship with your joint 15yr old? I'll assume he paid child support, but why did child and father not see each other and perhaps would not have even now if you had not reconnected with him 18mos ago?  Honestly, 2b-MIL may be afraid you will take these two youngest (your 2b-skids) and push her out of their life. She's had a big role in their lives. 

Not judging, just pointing out that there is a lot of possibles going on in 2b-MIL's head and she's not just being a b*tch but has genuine fears and concerns. Instead of addressing them with her (your 2b-Dh) you're fighting with her. It's probably time your 2b-DH sits down with her and talks and that he shows his mother he is serious about being a father, doing the parenting (not dumping it on you) and that he's feels now is the time to get the appropriate court/custody stuff done.

Set monthly routine Grandma/grandkids/stepgrandkids time (maybe one overnight EOW). Grandma gets her kid-time and you and adults in-DH get a date night. Too bad you just bought a house a couple months ago, because I'd tell you to buy one over a couple towns over and put a bit of distance between households. Dad needs to make it very clear to his mother that regardless of what she thinks or what opinions she holds, she is absolutely not to speak ill of any of the adults in front of the children. Not dad, not you, not BM... she keeps her two cents to herself. She's not doing these kids any favors by jaw jacking crap in front of (or possible earshot) of any of the children. It will not be tolerated. Poor behavior on her part will have consequences.