How do you plan a wedding when dealing with an opinionated future mother in law?

Well, since she thinks "sharing" means "asking her opinion", I just wouldn't tell or show her anything else. It's YOUR wedding, not hers, and just because you're getting married at the courthouse, you don't have to not feel like a bride. If she asks you about anything, tell her, but if she argues, just say "I'll consider that", then go and do what you want anyway.