You seem to have quite a few areas of tension with your mother.
1. The will. She can’t force you to make a will. It is a good idea to have a will, but you can make one without telling her, and if she finds out you do not need to tell her what is in it. Or, just make a will, then tell her afterwards, but say you are not disclosing any details.
2. The wedding clothes. Buy something else, but don’t tell her. Just turn up in it.
3. The nanny problem. He can call you grandma.
4. The phone. Ignore it. Don’t tell her what you are doing, just say you can’t talk or see her because you are busy, but you will ring her / drop in on her later.
5. The breakfast. You’ll just have to bite the bullet on this one. If she overfeeds your grandson, that is for his parents to decide what to do, just stay out of it.
It seems that you feel dominated by her, so you need to create some distance and autonomy. Set boundaries, and do not give in.