Tried this and got a 6 digit number that MDX also does not recognize. I made this reservation on Hotwire almost a month ago, so it's definitely not timing. I called back, explained it wasn't working, and the lady from Best Western confirmed "that is the number you need."
So I waited on hold with Disney for half an hour, explained the situation, and it's being escalated. Apparently, Disney's escalation CM now has to contact Best Western directly to get it sorted out. The CM I spoke with said he'd put a note in my account so that if it ISN'T resolved by tomorrow, a phone CM can assist me with making FP.