Plenty of posts on FT about how the passageways work. Crucial though is understanding that there are actually two of them, one for departures and one for arrivals (they’re not connected, except for staff).

Protocol is not to send passengers to C unless the flight is actually from there… but tech issues do happen. In this case protocol has always been to ask customers if they’re able and willing to walk, before directing them to the departures passageway. It’s perfectly legit to use this in either direction, you remain ‘clean’ at all times, and has been the case since the terminal opened. You end up in the lifts by A13. Option 2 if customers can’t walk is to ask HAL if any of the assistance buggies are available (these can use the passageway). Option 3 - very much least favourable - is to send customers to the train and then flight connections, specifically the customer service desk there. It is possible to reset the ready-to-fly indicator, but it’s not a simple process and takes a while to arrange. This also requires re-clearing security, so anyone with duty-free liquid purchases not in a STEB will need to have them refunded first.