I think it's important to remember that your mom is also going through a lot now - the end of the marriage, the anger of her children, etc. Please note that I'm not saying you're not entitled to those feelings.
She took a big step towards becoming a happier person and she wants to share that happiness with you and is craving your acceptance of her new life - a life she's been waiting to live - and she's reacting very emotionally to not receiving that acceptance because of her own feelings on the matter. She may be feeling a lot of resentment/blame coming from her kids for putting herself first in divorcing your father (thus the comment about putting you first for 20 odd years) and is dealing with that guilt.
The guy she's dating seriously overstepped boundaries (and his daughter, but maybe she doesn't know the whole story) and probably did it because he saw how upset/hurt/guilty your mother felt and wanted to bridge connections. He's probably heard from your mother about how upset everyone is and how everyone feels bad for your dad because he still loves her, etc.
I guess what I'm saying is that I don't see "creepy", I see defensive, supportive, clueless and enthusiastic. But not creepy. Your mother is going through a lot right now. She's found a lot of emotional support from this guy and is probably looking for reassurance that she is not a bad person or a bad mother for divorcing from a man who couldn't make her happy and 4 months feels like long enough to wait when you've been in an unhappy marriage for a long time.
You guys not wanting to move on right now is understandable. You feel bad for your dad because he still loves her, you feel bad for yourselves because your parents split, you feel resentment towards your mother from moving on in what, to you, must seem like such a short amount of time. But, a previous poster said you are recovering from the "loss" of your family. You haven't lost your family. Your parents split, but they're still there. You're recovering from a big change in your family.
Unless there's some history I don't know, your mom is not a villain.
I guess, what I would do is say, "Mom, I love you. I want you to be happy. This is a huge change for me and I'm still adjusting. Please understand".