You are in a difficult situation. First of all, no it isn't a dfing offense. There's a good chance a couple of the elders will say hi and nice to see you but they'll likely be too busy to want to talk about anything wrong you've done.
Regarding your mom, you're in a similar situation that I was in last year but with my wife. I hadn't gone to a meeting since beginning of January '17 and she was hoping I would go to memorial. My thought was that I'd love to go for her but what consequences would come of it. She would get her hopes up that I was coming back or at the very least that I'd be attending every memorial and the congregation would love bomb me. Yes, she was very upset and heartbroken that I didn't go but in the end I was very happy to have taken a stand. After that she realized that I was truly done with the cult.
This year she hasn't mentioned anything about it and she's ok with it. If I had attended last year, I would feel pressured to go again. When a person attends memorial it's like telling everyone "I still believe" and that is definitely not the case with me. Those are my two cents. I hope it all works out for you.