If you decide to stay I will tell you three things. 1. You will feel like saying f$&@ it and leaving weekly, if not daily.  2. Recognize you have to poser to change anything, and set boundaries to prevent being made responsible for someone else's responsibility's.  Your Bf will put kid duty on you, and BM will as well when she feels like being irresponsible, and the minute you accept you become their slave for life. 3. You WILL grow to love that baby like your own, but even though they may love you back they will ALWAYS....repeat ...ALWAYS want and love their Mommy more.  It is hardwired into our genetics.  Even when mom is a first rate screw up.  Even when you do everything for them and everything right.  4.  Your fiancé will make decisions with regard to his ex that you may hate and then end up paying for them (you pay as well) later.  I.e. she completes rehab and gets clean and he gives her custody back.  He forgives 11 years of child support and medical/dental debt, against your wishes and then she takes more custody and wants child support and him to pay ALL medical.  (Been there done that). 5.  Everyone, no matter who it is, will see you as responsible for the child while simultaneously finding you unemportant when BM enters the room.  If you can love with that, then you will be just fine.

First you need to own your actions and the consequences they had on her. You guys DID inform CPS and rightly they informed probation. You did the right thing. 

Secondly you need to own your own motivations. Yes, if she's a danger to the child the system has to get involved, but really you were hoping that she'd get locked up. I get it. I often wish my ex would fall under a train or my SO's ex would walk into traffic - I mean not really, but sorta?

Prison doesn't work - and the very best thing for this kid is to have a well and functioning bio-mother. 

Chances are she's not going to clean up her act and she will end up in jail or dead. But you don't need to do anything to hasten it. However, do inform CPS etc if you feel you need to - but only for protection of that kid. 

This kind of woman will be an unlimited source of drama for the rest of her life. Your SO is crappy at setting boundaries on her, and actively AVOIDS doing exactly what has to be done (restraining orders, blocking her on the phone, etc). The only way it can possibly be kind of tolerable is if your SO was willing to set iron boundaries around the two of you to protect you from her crap, and he isn't willing to do that. Your skid may be very lovable now, but wait until Mommy turns him against you and DH.

Just go now, find a man with no kids, or with a sane ex. They are out there.

This woman will never stop. I would suggest you get that restraining order for yourself. You are stepmom. You don't have to deal with this. At the end of the day, if you husband dies, you don't keep the kids. They go to her. Once you accept that, you can let things go. You have no rights or privleges without your husband awarding you those things. 

Get the restraining order, tell your husband to deal with his crazy ex, you are out of it and don't want to discuss it anymore. 

I don’t usually give anybody the “run from this” advice. But leaving is most definitely an answer. This is a mess and not your mess and too big for you to even help clean it up. It will stay a huge mess.

"I'm torn between my sanity and possibly making a huge difference in a child's life."

It's dad's job to create normalcy and a "huge difference" in his kids life. You can't come in on your white horse and save your bf and his child from this situation. 

Choose your sanity. 

My feeling is that you aren't so far "in" that you can't back out (actually we can all back out at any time.. but the earlier the easier).

At best his EX is going to be a constant source of heartache and drama in his household and for anyone that becomes a part of his household.  At worst she is a dangerous person that could cause you actual harm.. physically.. emotionally.. professionally.

There are a few others on here who have dealt with or are dealing with an addicted EXBM... it is a no win situation in almost all cases.  DH won't set boundaries.. even if he tries.. the BM is so messed up she can't/won't follow them.  The court system doesn't want them in jail.. costs them money and there is always this bottom line with CPS in that they will try to keep kids with bio parents.. even crappy ones will get chance after chance.

Your BF obviously isn't willing to go to the mat for his kids and ensure that she is not able to access the kids while her life is a mess.  Supervised visits.. no visitation.. conditions for access.

The BM has allies who are more than willing to do dirty work for her.. that means that you truly may never know where the next hit may come from.

Look, he may be attractive.. or a kind of nice guy.. but there are so many guys out there that can offer you a life without the danger and drama that this guy allows to be brought into his home.

You can't save the kid.. that's dad's job and he can continue to call cps.. your presence or absence will do zero for that kid.. dad is responsible.  Dad also had the kid with the crazy lady.. he needs to be the one to deal with the fallout... you can't save him.. you can't save the kid.. so save yourself.

Your SO is still making excuses for BM.
She is harrassing you and SO doesn't think he needs to block her?
What does BM need to do for your SO to protect you from this abusive behaviour?
Do you think marrying him will make him more likely to protect you?
I can tell you the answer is no.

All in can mean all out.
Listen to your heart and leave.
This isn't a healthy situation.
With time it will get worse.
Is this what you want for yourself long term?
Is this what you want for your children you may have with this man?

Your SO shows no capacity to properly handle this situation.
Do yourself a favour and save yourself from this situation: leave.

he is not setting boundaries with BM.  That a major problem in your life. And will continue to be a problem, forever. BM will likely died someway,  also remember, crazy BM. Make crazy kids 

I'm torn between my sanity and possibly making a huge difference in a child's life.

Not a valid reason to stay with a boyfriend. Even if you wanted to make a difference in the child's life, how can you do that if you are going crazy and are exhausted with all that is going on with the crazy BM? Resentment will start to build and that won't be good for anybody.

You are not responsible for this child, his parents are. His dad is the one who is going to have to shield him (and you) from the BM craziness. Is he willing to do that? It doesn't sound like it.

I was in a similar situation with a crazy BM constantly harrassing my SO, and he would engage. Or he would block her for a week, but then unblock her because of the kids. I've recommended this to others, and I'll recommend it to you- Get the Our Family Wizard App!! As the only form of communication. You can then block her on everything for good. We have it in our CO, because I told my SO I could not deal with this constant harrassment and intrusion, and he got tired of it too. We went from multiple texts and calls every day (from her boyfriends, also), to maybe 2 messages a month on the Family Wizard app? Changed our life. I don't think we would have made it had it continued. Now we just occasionally rip on her between the two of us, but she doesn't cause issues anymore. The courts can read the app if they need to, so that's a deterrent from crazy harrassing texts. Best $100 we've ever spent.