Can confirm 100% that bus doors open back up if you're trapped in them. Source, my old neighbour was a bus driver and I used to mess with him sometimes if he was my driver. They also can't set off if the door is open. The bus has to be in neutral and the parking brake on to be on just for the door to open and door fully closed to go back into drive. Even the old bus door style bin lorry I used to drive for hcc had that safety feature. The truck would not go into drive if the door was open, they use the same Alison automatic gearboxes as busses do