Good news, I got it!!!
But first, here's advice for you both: Keep trying, the $50 sale may(?) continue through Sunday.
What finally got my attempt approved is that I tried specifying the phone number of an alternate eligible cell number on my plan when supplying login credentials
(not my own cell #). I didn't expect it to work, but it did. I also needed to set Firefox to accept 3rd party cookies
(only relevant if you've turned 'em off).
Cust svc finally picked up right as I submitted the order and confirmed that it did go through and that:
- The phone arrives unactivated and can be activated by VZW on any line in the account
- Yes, if you were Unlimited beforehand then you will still be afterward, even though you're forced to select from a list of plans that doesn't mention Unlimited
- The system will accept tentative orders despite being out of stock and they'll be confirmed in order depending on how many previous orders turn out not to have qualifying VZW accounts. This didn't make much sense to me, but that's what she said.