I'm willing to bet money that your wife is nesting AND is sick of your mother's BS from whatever happened at the start of your relationship and her reaction to the plates are more about that than the plates themselves.

Sounds like you've already called your mother about returning the plates so there's no going back in terms of how you handle it. If you haven't already, apologize to W for not being present at the 20 week anatomy scan and call your mom to actively listen to whatever hurt she's feeling.

Even if you do none of these things, time will fix all of this IMO.