Dealing with MIL name

Hi,
I am new to Oh Baby - I have a beautiful two month old boy.

My question is to other mums dealing with their MIL...

When I got pregnant I thought she would be a great help and a source of support for my partner and I don't have my own mother to help. She was enthusiastic about becoming a grandma and offered to help when we needed it. She also promised to limit drinking around bubs etc. Before baby was born I asked her in a message if she would wash her hands after smoking as I was asking this of everyone. She got incredibly offended called us names and nasty things and cut herself off... Since then she has only seen my son once for 5 mins.

Anyway that's an issue in and of itself. My main issue is that she wants my son to call her something similar to grandmum... My partner and I agree the name she wants is stupid and we aren't going to encourage it. I wanted my partner to sit her down and say no please just be nana or grandma. She thinks the name she has given herself is cute. But because she got so offended last time we spoke to her, we are reluctant to address the name.

I'm really not keen on mum being in her name as I feel that I am mum and why does she need it in there she is the only grandmother my son has?

Do we ask her to give up the silly name or leave it alone?

TL;DR

My MIL wants my son to call her a stupid name with the word mum in it. Do I leave it or say something and risk her cutting herself off again?