Your life is yours to live. If you have doubts about marrying this man than you need to re-evaluate.

if the only issue is the BM casing problems and putting the kid in the middle... that is truly sad. Block BM from all your social media! 

You don't have to be a mom to your sk... but you can be a friend. All kids will want to be loyal to the bio parent... so it may not be that BM brainwashes kid into not liking you (although it's possible) but it could also be that the kid feels they are being disloyal to mom if they have a good relationship with you. 
If you love your SO and want it to work.. don't force anything with the kid, just let it develop organically. But make sure your SO backs you up when it comes to house rules and respecting adults! You don't have to parent the kid but that does not mean allowing them to be disrespectful.

So does he want to marry you because he loves you or because he needs a mother for his kid?  Before you marry him, that is a question that you are going to have to get answered to your satisfaction and then straighten out which woman he wants to keep happy - you or the BM.  Telling her what she wants to hear is just being a chickensh*t.  It is all well and good that his kid needs a mother but she has one.  That he chose badly when he procreated is entirely his fault and he is the one who needs to find a way to fix it without continuing the ripples of consequences into innocent people's lives.

He already chose who the mother of his child is, and it wasn't you. There are no do-overs in choosing the parent for your child. Your BF is both selfish and crazy for thinking that 1) another woman wants to step in and be full-time mom for his daughter, 2) that the BM wants another woman coming in and taking over as mother, and 3) that he gets to unload his parenting responsibilities onto someone else.

No is a complete sentence. If you don't want to act as a mother to this child (totally understandable why you don't), then don't. My guess is that BM is slightly losing her marbles about you because your BF is trying to push her out. Tell your BF no, you won't play mommy, and if he has a problem with them, he knows where the door is.

Don't get pulled into doing something you don't want for some d*ck. There are plenty of other men out there, and being single isn't some status of shame. You have ALL the options in this, so use them. He doesn't control you or your life.

Soooo... Saying he "needs a mom for his kid" is a red flag to me. I LOVE the skids. But, i admit, the more I've given up for them, the more frustrating it gets...  No matter how involved you are, no matter how much you get to genuinely be a parent, there are ALWAYS going to be blaring things that will happen, either with your DH or BM, that remind you that you're this weird third. And that ultimately, you don't get a say in things unless your DH gives you an okay.

The BM is already out to get you, and you shouldn't be afraid of what to post to your social media.

Just decide what you genuinely want!!!

BF is placating the BM yet wants YOU to have 100% responsibility and 0% authority is a huge red flag!!

RUN while you can!  you have been WARNED.

He says he needs a mom for his daughter because his bm is not a good mom.

If BM is not a good mom, that is not your problem. He will just have to double down and be a good dad but he cannot expect YOU to be his daughter's mom.

My advice is:

- Don’t ever ever ever ever EVER marry this person. 

- Lock down all your social media to “private”.

- Remind him that his child has a mom.

Red flag with this man wanting/expecting you to substitute and play mommy for his child. This kid has a mother and he's going to have to live with and accept that he reproduced with someone that is not up to par. Not your problem so he needs to get it together, learn to parent effectively and have no expectations of someone else stepping in to do the hard work for him. 

He doesn't get to pick and choose who is the child's mum. The child already has a mother and you have absolutely no obligation to pretend to be another. 

Next time he starts with htis crap tell him you'd rather be a lover, a wife, a partner than a pseudo-substitute "mother" for his offspring. 

Frankly, if anyone ever told me that they wanted to be with me because their child needed a mother because their own was not good enough I'd run so fast you wouldn't see me for dust. Yuck. How totally unromantic and unloving.

Don't you want to be loved for you?

If she’s not a good mother then he needs to be a better father. It’s not up to you to be the replacement. I have a hunch he wants you more involved so he can shove all his parenting responsibilities on you instead of himself. 

You can be a kind loving and supportive partner for him, without having to be a Substitute Mommy.