Is this a dramatic request? That my grandmother's silver platter only be used to serve treats within our home? Is this a good boundary?
Yes, to a WAW it is pretty dramatic. And, yes it seems controlling to me, unless you would react the same way if she wanted to take the platter to some event you had been invited to (like relatives). But whether or not you would, isn't really the issue here. You just make yourself look rather foolish in her eyes. If you had discussed not ever using the platter outside of your home, and then she took it anyway, I could see you making a bigger deal about it, but this isn't what I consider "boundary" material when much larger problems are at work. A boundary in a R isn't so much about which platter to use. It's about fidelity, truth, honesty, and respect. Sure, you can make your request about the platter known....but don't call it a boundary. A boundary suggests a line not to be crossed, a confined area to stay within, and/or consequences otherwise. The traditional wedding vows is a good place to start with the meaning of boundaries, however, it doesn't state the consequences. I guess it would be obvious or there'd be no need for vows to be spoken and a licensed person to pronounce them as Mr. & Mrs.
You did keep calm, and that's good. But as MWD says, choose your battles wisely. Only you can decide what feels disrespectful to you. In the future, if something like this happens, tell her that it makes you feel disrespected when she knows where you stand. (Make sure she does know, however.) When you have a WAW in an A, it causes you to be hypersensitive about everything, including what belonged to grandma.
IMHO, you need to stay in Newcomers.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!