The only thing you can really do is disengage from things having to do with BM. 

Harsh truths:

1. As a step parent you aren't entitled to any information. We have no legal rights, only what our spouses grant and give us- and even then if the other bio parent decides to block things, like school info, they might not succed but they make it super difficult. 
2. As a step parent- you will always be 2nd to the bio. Kids are wired to love their parents and if BM sees you as a threat, skid will see you as a problem too. 

If you step back, let your DH deal with it and stop being a part of it, your life get's easier. Nod and give him a compassionate smile and say "I know this is hard... but anyway, I was thinking next Saturday we check out that farmers market I have been wanting to go to!" Just acknowledge but don't offer solutions, don't let yourself get invested, just change the topic. 

I pretty much agree with what JMTB has to say here.. she has the poster child situation for horrid high conflict and PAS with abysmal legal results.

The thing that caught my eye was that you are having "other issues".. and that it's just kind of piling on with the BM and skid issues.. 

Bluntly, Steplife is hard.  You have no control over how BM sees you, reacts to you, and precious little you or your DH can do about what she may tell the child.  Of course, child should be properly educated about truths in age appropriate fashion.. but it's unlikely that anyone can make BM be nicer. 

If you are finding yourself unhappy with your partner AND steplife.. I would be looking for an exit.. only super strong relationships with people that can have thick skins tend to make it to the other side.

What does their court order say about their schedule?

Disengage from BM. Dont talk about her or worry about her, dont ask about her.

If SS is acting up because of things she is sayong, your DH needs to step in.

You two MUST be on the same parenting page. Be a team. Consider counseling.

Read around on here, you are not alone in your feelings and experiences.

I think you should look at the BM ignoring you as a positive. Lol

Also, she probably notices that her behavior bothers you, so she continues to do it. Believe me I know, because I have done it as a last resort at work with people who didn't understand or respect my boundaries. I'm not saying that is you, but what she is doing is a tactic to use when you want to upset someone you constantly see.

I think once you show her that you don't care her behavior may change. If not, who cares? She doesn't pay your bills. 
 

Let SO handle drops off and pick ups. No. Need to have anything to do with BM.  

Does SS complain about those weekends with his mother?  Does DH put him on notice for any bad behaviors/talk after those weekends? 
 

Disengagement from BM is wise but if DH is not correcting any negativity then you will have a bigger problem as SS ages.  Does she have the money for court?  Does DH confront her about games she is playing?  He's had custody for a long time, must be a good reason?   Her past behaviors are a good predictor of future behaviors.  She obviously thought DH was still enamored with her but you married him and destroyed her fantasy.  Now she's pissed about that and using SS as her proxy to cause strife.  
 

Your DH needs to handle it all and protect you from any drama from either of them.  He needs to follow the CO and force BM to follow the CO exactly.  It's there for a reason.  Does he use an online wizard app for communication with BM?  Does he jump every time she texts? Or does he make her wait until he's ready to answer ?   Little things like this can really undermine a marriage.   Are there better ways to draw some boundaries with BM and minimize the drama in your daily life?  

Anyone acting in loco parentis can assume responsibiity for the well being of a child.  I never asked permission. I just did it. I disciplined, parented, engaged medical care, engaged schools as a parent, supported, etc... and no one ever said a word or asked a question about it.

An SP can readily get delegated power of attorney to parent from their prior relationship breeder mate (verify with your State what is required).   I never asked, I never asked for nor was I ever given formal POA as a parent, nobody interfered in my being a parent to SS in any way.  As equity life partners my DW and I have always been equity parents to any kids in our marriage, regardless of kid biology.  As it turned out, our son, was my SS. and an only child in our marriage.  We married the week before he turned 2yo. He asked me to adopt him when he was 22.

I have always done the smh when SParents do not fully engage on parenting underage Sspawn in their home/marriage. I would have never done anything less.  My DW would have accepted nothing less.

My DW and I raised SS as a married couple, to our standards, protecting his best interests from the shallow and polluted end of his gene pool. The genetic cesspool that raised SS's three younger also out of wedlock half sibs by two other baby mamas to be on the dole (Spermidiot Spawn #2), in prison (Spermidiot Spawn #3), and closely following the convict (Spermidiot Spawn #4).

Legally recognized rights or not, I took every right needed to be SS's dad.

Courts can declare that a SP is "not a party to the case".  Two judges told me that. I was only allowed to comment and participate in the hearings when I was on the stand. As I was not a party to the case, I refused to provide my income information which resulted in a threat of being held in contempt. I would then ask the Judge if I was a party to the case. The Judge would repeat that I was not a party to the case. I would point out that as I was not a party to the case, neither was my money.  Ultimately I would provide my income, but only after making my point.

No Judge ever said I could not parent, care for, support, etc... my SS.  Just that I was "not a party to the case".  Even if a Judge were to rule that an SP cannot parent, support, care for, etc..., which no Judge would do would do in all liklihood, once the hearing is over and everyone leaves court, the Judge is not in the  home, or present for daily life.   The odds of the Judge saying or doing anything to interfere in Sparenting, are just about zero. Even if the Judge were to make that ruling while in court.

Beyond custody, CS, and visitation schedule... Family Law courts have limited ability to engage in the daily lives of families, marriages, etc...

IMHO and experience of course.