I think your step daughters clinginess has far more to do with her father leaving,than her mother not moving on.
It's very typical behaviour in girls with absent fathers.

I think your own problems with the parents and her relationship are causing you to miss the obvious.

It's not ideal, but I think it's really good your husbands daughters have their grandparents around, as this isn't always the case after a separation, and even better that the relationship is amicable. This is a much better situation than most.

I am sorry this is upsetting for your husband and yourself, these kind of battle lines are hard to erase. You need to shift your focus.
This is between your husband and his parents. NO One else, and nothing good will come of trying to point fingers at the ex.
Your step daughter is afraid of losing the men in her life, because the most important one left her. NO other reason.
In fact I would have thought her mother's behaviour would have taught her, no matter how long she holds on. It changes nothing.

These issues need to be addressed separately. Your husband needs to speak to his parents, alone preferably, since you weren't involved.
And his daughter needs help for her abandonment issues.
Only she or her legal guardian can sort that out.