Continued from part the second.
And now for the third update.
Without the ability to run ads to my shop, I'm pouring money down the drain on a fancy website that no one's buying from.
So eventually, I decided that, since Facebook has no interest in resolving the problems with my account, I would delete this account in its entirety and start fresh.
From within Shopify, I tried (once again) to disconnect the existing link to Facebook. Again, it wouldn't let me because I need to be authenticated by Facebook in order to do so.
I managed to contact Facebook Support from within Shopify. They said they really needed to speak by phone. I agreed as I really just want this over with.
I spent 40 minutes on the phone with a Tech, who (to be fair to him) really did come up with some creative problem-solving techniques.
Alas, in the end, he was unable to resolve the problem.
I asked if he could just disconnect the account from their end. Nope, not without the consent of the authenticated FB profile owner (i.e. me).
So now, here I sit, pouring money down the drain on a business I'm unable to advertise.
Continued in part the fourth.