Welcome LittleMiss and OhNoReally x
Little Miss, no wonder you're having nightmares - this man is threatening to invade your safe space. But it's good if it's encouraged you to come on here and hopefully find the help and support to say no to him and to your Nan. She is as much an abuser as he is, because she continues to mentally abuse you and force you to have him in your life. I know you're clinging on to the idea of having some family, but she will never be the family you need.
Can you confide at least a little in your partner and ask him to be strong for you and to tell your Nan that she cannot come?
I think it would be really helpful for you to read Toxic Parents as this is what your Nan has been to you - it may help you to see through her cruel and manipulative treatment and give you the strength to create healthy boundaries.
OhNo - I read your other thread, don't beat yourself up for not being able to stand up to this creepy guy. There's a part of you that was damaged at 12 years old, and that part of you is stuck at being a frightened little girl who doesn't have the capability or support to stand up to creeps. When someone like this man triggers the same reactions as your abuse did, it's natural to panic and to freeze. Give that 12 year old you a hug and promise her you'll look after her and won't let anyone hurt her.
I only got as far as you told your DH and he just said 'great'. Has he responded any further? Maybe you need to spell it out to him - really break it down to the 'ABCs' of what's happened and how it's affected you, ie: "This guy has really frightened me; I don't feel capable of standing up to him; I need you to be strong for me and tell him in no uncertain terms to fuck off."
If you can't get that support from your DH, PM me the guys email address and I'll tell him to fuck off for you, if you like! What a wanker.