[mothered]

the XM3s are basically brand new other than the fact one of them has a piercing screech when they're on.

Perfect, It's good to go.

My biggest issue is what might happen after they have accepted it and whatever it is they do with it back then.

Who's to say that one of their employees didn't swap the Item?

Believe me, there's no cause for concern. You're simply a customer who's (seemingly) legitimately returned the original purchased Item.