Regarding the passcom, do drivers really wait to the next station to answer it? This seems a potentially dangerous gap.
TW1 tells us to make contact with the person if possible, my TOC says to only do this if it is safe to do so. Generally speaking that means to not do it on restrictive aspects, or approaching the braking area for the next station stop. On my route that means there are very few places that I can attempt to make contact before hitting the next braking area. On a route with longer gaps between stops that balance changes. Contacting a person using a flaky passenger alarm is a multi step process that is a distraction and that needs to be weighed against the risk of potentially having an incident like a station overrun, SPAD or over speed.
The only time we must immediately stop is within the confines of a platform when departing, and our stock does that automatically within 120m of a station stop where the doors have been released.
I can tell you in 15 years and around 100 combined, passenger alarms, call for aids and egresses, only one warranted an immediate stop and that occurred while departing a platform, a small child was separated from their parents and they didn't realise until after the doors had closed.
The vast majority are kids messing around, people that have missed their stop, or someone reporting that another passenger has fainted because they didn't have any breakfast.
As for if a civilian were to use a two arms up hand signal to try to stop a train, or waved a phone light at me, it's never happened, I'm not entirely convinced I would register it without another indication that there was a dangerous situation.
Would the emergency door egress handle not initiate a brake demand that cant be overridden or is that cancellable too?
As another commenter said only at very low speeds, on our trains that's around 3mph, above that the doors stay locked and like a passenger alarm it shows us the nearest camera and gives an alarm in the cab.