An update on the case. ..
I called GoDaddy again after posting here, and was told basically that the domain wasn't listed in their "back end" for Office 365, so if Microsoft's system was saying it was associated with another account (even if it was with them) I'd have to call Microsoft.
I then called Microsoft and got passed between a couple of departments, because there is no department for technical support for people who don't have an account with Microsoft itself. In the end, they couldn't do anything about it either, because it *was so* on a GoDaddy-associated tenant and GoDaddy are the only ones who can remove it.
I then contacted GoDaddy sales and asked to purchase Office 365 through them, without mentioning any of this. Two minutes into that call (before the sales guy had a chance to set it up) I decided to check again to see if the domain could be added to my other O365 account, and lo and behold, it could! I told the sales guy to forget it, and sorry for wasting his time.
I'm not sure who fixed it. I doubt it was the sales guy, because he really didn't have time.
Microsoft should have support available for when a reseller isn't resolving a problem. If GoDaddy won't remove my domain from a tenant (for whatever reason) Microsoft should be able to force it. This should never have happened.