[JXN]

they might be able to lock the apple-account-related functions though, couldn't they?

Correct, they do have the capacity to do so.

I did end up sending the SN, I didn't send a fake one just so I could avoid any complications along the way, worst case they lock the watch and i get my money back. The Team member told me the reason they asked for the SN is to simply verify with apple that the apple watch has not been activated which it has not. Hopefully apple do not lock the watch.