This sounds like a bit of a mess, but one that you should be able to get sorted out.

Do you have a booking confirmation email showing the full itinerary? If so I would do as you suggest and ask at the ticketing office in MIA, giving yourself plenty of extra time. That said I don't know what powers the BA ticketing desk there has, but at least they will fully understand the issue face-to-face.

The note about you requesting deletion of the first leg is very odd, not least because that would ordinarily involve a reprice to a much higher fare, which obviously you didn't agree to. I'd therefore be fairly confident that someone will eventually look past whatever that note says, work out what the problem is and sort it out for you.