Ok so in situations you mention above are there approved methods of preventing a trap and drag incident?
Yes indeed. The simplest being, simply look back out of the window. This relies on having a straight platform of course, which is often not the case.
For a curved platform (or a train with more than 8 cars), it's bell the signaller up, key off, walk back to the last coach and take off the door release, walking coach by coach repeating the process back to the front of the train. Key on, DSD test, bell the signaller up to get the road back - easily 10 minutes delay.
Where it falls down is curved platforms during the rush hour where the enormous numbers of people getting on and off the train make it very hard to tell if that person standing right next to the doors eight coaches back is trapped, or simply standing close to the edge.
It is a far from perfect system - no driver would ever leave if they are unsure whether someone is trapped as the stakes are far to high. Job loss, a death on your conscience and a potential prison sentence. But take into account 100 station stops per shift, fatigue and the inevitable human error and mistakes can and do happen.
There is also a strong suspicion that blind spots can occur on monitors, which you will not be aware of unless you count each set of doors on your train (20 on a 10 car).
Quite a few locations on my network really should have bat and flag. The reason they don't, of course, is down to cost.