This looks like a pretty extensive list. I don't know what preceded this because I'm not famliar with your story (I can go back and read). Will there be retaliation (using the grands)? If so, would you consider doing this slowly. . . perhaps first stopping the salon visits and responding only to occasional texts--or do you need to rip off the bandaid right now? 
I slowly disengaged from my skids, so it was sort of like boiling the frog(s)--but it was only because I kept giving them chances and we hadn't gotten anywhere remotely near the 20-year mark. We are at zero communication now, a lovely place to be. 
Do you have to be around her? 
 

I'd say yes to all of the above.

If your dh wants an explanation,  tell him that she never welcomed you into the family when you so badly needed to be accepted, that she resented you, that she (whatever else she accuses you of, but from your point of view), that you have given up. You've had enough. It's not going to happen. It's been X years and she still hasn't warmed to you despite all you've done for her.  You're done.

Maybe add that if he and she let this go, you will abstain from writing scathing reviews (which you would,  obviously,  never write... but they don't know that) about her salon.

the salon visits, find a new hair and nail person/s.  Stop answering her text.  Block them,  Don't open them.  Phone is screw up. You will have the someone look at it someday.  Don't know why you interact with the ex in-laws.? Unless graduation, or something. Once a year 

I'm new to your blogs so I apologize if I'm incorrect or out of context here. Are the grandkids Starry's kids? If so, disengaging from one but not the other may not be possible. Many parents feel that if you don't have a relationship with the parent, you don't get a relationship with the kids. Starry may withhold the grandkids if you don't maintain a relationship with her. 

Disengagement from the skids- trickles down to the grandkids. Once I stopped planning things for "them " it was more difficult to arrange anything involving the grandkids. It's been five years now- they've gone from teens to young adults and DH and I have zero relationship with them. DH still tries- I stopped years ago after seeing apples don't fall far from trees in our case. 

I think that your boundaries and list is excellent. You have a reason. Stay strong and stick to your boundaries. It will be helpful to disengage and I feel like you're going about it in a good way that will benefit your life greatly. I agree with the others that it will be messy with the grandkids however I think you're doing what needs to be done to get rid of this toxic person from your life. 

Just off the title alone....20 years is a prison sentence 

Free yourself!!! Don't voluntarily "incarcerate" yourself anymore. 

I will say this...from experience....she WILL use those grandkids as pawns to hurt you, so gird your loins. With that said, you can implement your new boundaries all at once. Personally, I would not even  be on her social media pages. Remove yourself and block. I guess ignoring calls/texts can work but for many years, my DHs kids were blocked with zero access to me. They are now unblocked but probably don't know that. 

I'll add...I am slowly allowing DHs kids to be around me again. Six plus years of being banned from my life has clearly shown them my stand....and DH has told them (or I hope he has), if they choose to bring any old behaviors with them, they will be banned again. I've been around two so far....it was very superficial. Nice but like dealing with a stranger. Or as I like to say, interacting with the cashier checking my groceries. 

I dealt with crap for 12 years....you've been in this for TWENTY YEARS!! As someone said, that's a prison sentence. Take your life back. Put this grown ass woman in her place....and take back your peace.

Best to you.