OP, I think that you are borrowing non existent trouble. The daughter has made it clear that she does not want the yet to be born grandchild around you. That should be your standard. You will both share and respect the daughters wishes. Your partner is free to visit and spend time with his grandchild whenever and wherever he wishes with the exception of your home.
Essentially, nothing changes for you. You were not spending time with the daughter, and now, you will not spend time with daughter and grandchild.
Share his joy in his having a grandchild. A new life and continuity of a family is a positive thing and should be recognized as such. Beyond that, you have no role to play in the daughter's crazy show.
You are his partner, but you are neither step mother nor grandmother. Everyone involved is aware of those facts. Do not assume or let them try to impose those roles upon you. You are simply dad's partner and that is the only role that you wish to play in this crazy drama.
If your partner wants to have his grandchild child over for the occasional sleepover, he might consider purchasing a small condo. While that seems to be an extreme solution , it might give him the freedom to entertain his daughter and grandchild without imposing upon you.
His love for daughter and grandchild will never be conditioned upon their treatment of you or anyone else. That love is tied to their familial bond. His willingness to impose them upon you should be however very much conditioned upon their treatment of you.
He should make arrangements to spend time with them away from you. It is obvious that you blame the daughter for your not being married. That is not the daughter's fault; that is a decision that you and your partner made together.
Recognize that with the birth of the grandchild, little changes in this situation. It is just a case of yet another character being added to the existing melodrama. Having your partner spend time away with them will give you freedom to spend time with your own friends and engage in your own hobbies. Some alone time can be a good time to become reconnected with yourself as an individual.