I frankly feel that the longer you let difficult people get away with being difficult, the more difficult they become.
That said, in your place Gigi57 I would follow the advice already given about talking to your son. Point out that you love him and your grandson and are quite prepared to love your daughter in law too, but that you feel she doesn't give you the chance.
I would also say that you feel it is unfair if you are to be banned by your DIL on account of an argument between your one daughter and her. You neither can nor should be expected to take sides in a disagreement between two adult sisters-in-law.
Unlike others I think I know what bit your daughter on that famous occasion. I see no reason why she or anyone else should put up with behaviour that she dislikes, but if either party is trying to rope you into their disagreement, tell them it is not any of your business.
To be fair, your DIL may be expecting you to take your daughter's side and be pleased to realise that you have no intention of doing so, so please make that clear to her and to your son.
Your DIL's mother wasn't a great help, was she? Again I feel sorry for your DIL - unless she really is and always has been a pain in the neck her own mother's attitude seems unjustified.
Even if the young woman is a right pain, whose fault is that? Hers, yes, but to a great extent also her mother's!
You neither can nor should attempt to bring up your DIL, but see if you cannot get so far that you can sit down and discuss the matter nicely with her. Otherwise the chance of you being "allowed" contact with your grandson seem very remote to me.
Talk to your son and do, please, make three things very clear: you are not going to get involved in whatever is going on between his sister and his wife; you love them all, including your DIL; you want this just to be a blimp on the radar and to be part of your DS, DIL and GS's lives.