Since you're able to log into the EA account and there's no ban or suspension on the account. Perhaps the six month wait restriction is preventing the account unlink.
According to EA there's only four restrictions that prevent account unlink.
- Bans
- Suspensions
- Disabled account due to inactivity
- Six month wait on newly linked accounts
It would suck to wait six months and still not be able to unlink the account because something else is preventing it. Unfortunately there's no error message specifying why the account unlink is failing.
I'd keep asking EA support to unlink the account. If they can't do it then that leaves waiting six months or creating a new Xbox account.