If SD or BM-masquerading-as-SD texts your DH again, he should respond with, "I'll be happy to discuss any issues you have in person."

My DH never responded to hostile skid texts with anything other than "we'll talk about it". 

And be careful in any (if they ever happen) one on one meetings. DH's last attempt to discuss OSD's delusions that she was under duress in our home (oh, how those rules and standards of behavior hurt) in person was a complete set up. She refused to listen, to see the facts when DH presented her with the texts and emails and calls to her after she told him he "never tried to contact her", and was generally verbally abusive to him. He still has not recovered from that meeting nor the last phone call with her when she screamed at him. 

He's still sad, has some bad days, but a therapist is helping. I hope your DH keeps his cool and continues to be the bigger person.

BM1 texted my husband this directly without impersonation and she also voiced it in multiple convos over the phone. She told him that he was not prioritizing his kids and if I wasnt going to "get along" with OSS22 and her other son, either he should divorce me or accept to never see the children again. I repeatedly explained the issue isnt getting along because we arent friends, i am an adult, the issue is refusing to follow rules. Then I was accused of wanting them to be "robots" for asking normal things like closing the kitchen cabinets after using something or rinsing your bowl/dishes after you use them....Apparently its "robotic" and I should just let them live!

I always told my husband that his children came first and he could divorce me if he wants to focus on parenting his children with behavioral issues but he refused (would probably have been too hard and miserable without me shouldering the brunt of the problems and being the scapegoat for their entire family)

OSS22 was an awful human being but BM1 always wanted to make him seem like this innocent child. Every time he would do something, both parents would say "he is a child!", even when it got physical

The bottom line is only your husband knows what he wants and she can tell him wtv she wants directly without using SD but she knows your SDs communication holds more power and guilt trip. What you need to figure out is where your husband stands. Whether you are secure or not with your husband does not matter, what matters is how easily manipulated/swayed can your husband be by his ex and SD? If its easy, then consider your days counted. 

Also remember he probably needs you more than you need him so he wont leave you