Lilac, its clear as this develops page by page you do not have a good relationship with MI. She irritates you, asks personal questions, is intrusive, you don’t feel as important as your husband.. Its what you have said. If it were me I would talk to her but not in a combative way. Only you can decide but you will probably get your husband to do it. This could very well cause a rift because obviously having two children of his own, they come first. If it were my son he would say something as our bond is strong and if his wife didn't want anything to do with me and wouldn't talk directly to me he would then keep out if it, its not his problem, he would continue to see me.
Would you want one of your children, when they are older and you didn't get on with their partner, be happy to see them go because of it? Its how she is. Maybe let him choose her presents in future.
You have asked for opinions and that is mine, others may well differ.