(1) The question pre-judges the outcome of an active investigation - nobody yet knows what may or may not have prevented that crash. We don't have mirrors or cameras because we have devices that indicate the position of flaps and other systems, as well as take-off configuration warnings. These are very effective, and over the millions of flights that have operated since their introduction, you can count the number of crashes that have occured on the fingers of one hand.
(2) I don't work for BA but I very much doubt it is closed before boarding. Before engine start, yes, but there are people like cabin crew and dispatchers who need access to the flightdeck before an aircraft can depart. More likely the crew were busy and that was the first explanation that came to hand. Unless we're extremely busy, we're usually happy to have visitors to the flightdeck before or after the flight.