Sadly, in my opinion, there is no etiquette for dealing with former in-laws. Often, in-laws love and care about their children’s former partner even when the relationship ends. Navigating the continuation of the relationship with their child’s former partner is challenging at best, especially if the adult child sees it as some kind of disloyalty. The in-law doesn’t want to be caught in the middle of the couple’s argument but does want to let the former partner know that she or he is still loved.
It looks to me like your former mother-in-law is struggling with that very problem. She doesn’t want to “take sides” or be a go-between or a source of information about her son but she does want you to know that she cares. It’s difficult, but she keeps trying. So I think you should accept the love that is behind it and not expect her to tell you how she feels about her son’s behavior.
Meanwhile, when you are feeling stronger, I hope you are talking to an attorney about how to make the divorce happen. The stress of being in the “limbo” of neither married nor divorced is not helpful in your recovery or in your ability to move on in life.
I wish you well.