If you want door opening times to be the contractual method of recording delays, I suggest you campaign for it.
If by 'recording delays' you mean the arrival time that counts for Delay Repay, I don't need to campaign for anything. It is already the legal position, as recognised in aviation law for example (where EU/UK261 similarly don't define the 'arrival' time). When a wrongly rejected/underpaid claims is appealed, TOCs typically accept this point.
If there is anything to campaign for, it is the use of more accurate data to decide claims in the first instance.
To me, arrival is the time the train comes to a stand, not the time i alighted from it - would you claim an extra 2 minutes because a lot of people were in front of you with luggage ? (I suspect some on here would try it).
The time the train comes to a stand is a complete irrelevance as far as the passenger is concerned. They cannot alight it yet at that point give the interlocking on modern trains; there may have been more mileage in such an argument in the days of non-CDL slam-door stock but that's long gone now. It is the time you are first able to alight that counts.
The fact that you're talking about other people impeding your exit and suggesting people "would try it" - none of which I suggested, nor do they have any bearing on Delay Repay - makes it clear that you're just interested in supporting the industry's position no matter how unreasonable it might be, rather than looking at the matter objectively.
On many trains there will be a 5-10 second wait before the doors are released. On some occasions it will be considerbaly longer, e.g. if the guard has to release the doors but is completing a ticket sale or isn't at the right door in time. Even more so if the train is being coupled up on arrival or there is a technical fault which delays release.
i do not believe that there is anywhere within Delay Repay that stipulates how arrival times are
measured. Therefore the Reil industry will use internal systems for that purpose.
I agree that's what happens in practice but that does not mean that the industry data used is definitive. As mentioned above, if a passenger challenges the delay recorded (ideally with contradicting evbidence), the TOCs don't really have grounds to stand on and most accept such appeals.
If you want to get that changed, dont whinge on here - lobby for it.
The industry currently uses TD data because it's not interested in putting in the legwork to use a more accurate source, and most people just accept that. I agree that this should be changed but there is no point lobbying for it, you would have to design an end-to-end technical solution and market it to the TOCs or NR for there to be any chance of it getting anywhere.
I'm not in a position to do that, but I'm quite entitled to put across my views - as well as to explain the legal position - on this as it is a forum for discussions.