Don't recall anything on 13 being designated as such but the overhang and the struts to support the overhang can be an obstruction if trying to look up.
Without Key, you would probably board maybe 5-10 minutes later - that assumes you are waiting to board. Once boarding starts and the waiting area clears, there’s no advantage for Key (other than perhaps a slightly shorter check in line).
Most MDR tables are about an inch thick.
Location of the table might be an issue - on many ships it might be a bit too tight trying to get the stroller all the way to the table. Work out a workable solution with the MDR host.
They use the same login you created when you login to claim your package. When you try to login a fourth device, it will kick off one of the existing devices.