My mums nanna and nanny and her other nan calls herself nanny “Xxxx” (first name). Baby will likely choose Nana for my mum As her cousins have.
This would annoy me too but anything my mil does annoys me lol. Just say you don’t want to confuse her so what does she want to be known by?
Honestly if she's going to change it I'd pic for her. We have Nanny Name for my parents grandparents otherwise we'd get confused between all the nanny's we have (MIL is Nanny, plus both of my partners grandparents
My mother in law has been at this for three years now. She still can’t decide between Nanny and Nana. It is what it is.
I honestly wouldn't be too bothered about it. Your baby will decide what to call your MIL and that's what will stick.