As I say, what should be done 'by rights' and what is done are often two different things. I favour a consistent approach in line with standards, but ultimately if a TOC or TOCs want to go and do their own thing they've only got themselves to blame when someone gets hurt. Using the 22x example, if the Guard or the Customer Host were to come into the driving cab just as the train jerked, they could fall from height at speed and likely be killed. It doesn't make a jot of difference whether they were supposed to be in there or not, there would still have been a big massive gaping hole in the side of the train with a fall prevention system (that being, in this case, a door) which isn't being used correctly.
RAIB involvement for a passenger door open in traffic would be rare but it would depend on the individual circumstances of the incident, I recall a couple which RAIB investigated off the top of my head, one where a conductor nearly fell out when he tried to lock a door out of use at speed which then came open and another where a door came open but the driver overrode the brake application for a good number of miles. A typical door open in traffic incident wouldn't be reported to RAIB.
If reported to the box, that will then be passed on by Network Rail to the Train Operator who (are supposed to) record the incident. The crew should be following whatever procedures are necessary in the rule book/company instructions for the type of door in question, so for the crew the procedures will differ, but for the purpose of recording of incidents a cab door open in traffic is still supposed to go down as a door open in traffic (of course many cab doors these days are on the interlock anyway so it's a moot point).