Your nan sounds very rude OP. Has she always been this rude and obnoxious? My husband has a cousin like this; horribly opinionated and rude, and thinks she has a right to make comments about peoples weight, their home, their job or career, their car, the way they raise their kids, their garden etc etc. Yet God forbid you say anything to her. 
I am a wee bit puzzled as to why people think you should never discuss names of your baby with people before baby is born though. I mean, when you name your baby when it's born, people will still bitch about it if they don't like it, so you will just be offended then! At least telling people before gets it out of the way.
Also, it does give an opportunity for people to voice opinions on the name, and if too many people recoil in horror, it gives you time to rethink it. Not to change it because people say so, but just to make sure you are 100% sure about the name!
I mean, a neighbour of mine wanted to call her daughter Beyonce! (I am not kidding!) She got so many people saying 'omg no! (Friends, family, neighbours, siblings, the works,) that in the end she decided on Sophia! 5 years later, she said she is genuinely glad she decided against Beyonce.