I think your user name says a lot. You are not the mother of three baby girls, you are the mother of three women.
Your resident daughter may or may not be neurodivergent. Whether she is or she isn't you are not doing her any favours by babying her and treating her as an incapable child.
For her own self respect and development she needs to be taking some responsibility for her life. She needs to be contributing, financially and practically, according to her abilities. She managed to find a partner and live away from home for two years and only returned for financial reasons so her practical abilities must be reasonably high.
You need to up your expectations of her in every way. Saying you don't think she will be able to move away for years is almost like cursing her. You have no idea what's around the corner for her. When your husband left you high and dry with three little girls I bet you didn't envisage that just three years later you'd meet someone better - but it happened.
It's tough when adult children return to the nest. Our own DD returned after 4 years at uni overseas and was a nightmare. She certainly tried to dictate how we ran things and complained incessantly about us living our lives. We love her dearly but the sigh of relief when she eventually moved out was probably audible 2 doors down. But our relationship is stronger now for having successfully negotiated a very tricky time.
Your DP sounds very patient - where does she get off telling a grown man how much to he can watch in his own home?
Be firm, make boundaries for yourself as well as for them and then stick to your own firmly, no matter how hard your DD tries to suck you into the drama.