But in a lot of the cases I’ve seen it really has just been a poorly managed situation.
To be fair it does descend into utter chaos on a regular basis, unfortunately, so I can understand why you have formed that view. When things are running well it hangs together, but as soon as there’s disruption it’s really just firefighting. There are various reasons for this - one being delays getting the screens altered because the people controlling them sit a long way from the people working on the platform plans/interfacing with the staff during disruption etc. I could go on (and on)!
Why exactly do the doors have to be closed if there is no crew on board? Couldn't the previous crew boot up the reservations for the next working then just leave the doors released for boarding?
Usually the next trip is put in before arrival at St Pancras, and doors remain open between arrival and departure. However if the unit has come in ECS there will only have been a driver aboard, they have nothing to do with seat reservations system. It also won’t necessarily be known to them what the unit’s next working is so, unless otherwise asked by platform staff, it’s best to err on the side of caution and leave it locked, incase another unit is coming in on top to couple to it.
The other issue with 222s is that opening the doors before the trip has been loaded will mean the seat reservations don’t display correctly (or at all). Therefore it’s best to wait until the guard has boarded before releasing doors. You also need both a driver and a guard present as releasing requires a driver’s key on. Is it a stupid design? Yes - hopefully the 810s will be better, but it’s early 00s tech, and what has to be worked with for now.
And why is people standing on the platform not acceptable, when standing on the concourse is considered OK
Simply because the concourse doesn’t have a platform-train-interface!
Why this happens I’ve no idea, as the train PIS will be working so clearly some part of the train knows it’s headcode etc
The reservations are downloaded to the on-train TMS in the middle of the night AIUI but still have to be manually activated for each trip by the TM (in case of disruption, last minute set swaps etc.)
I've often found the EMR St Pancras staff to be rude and unhelpful. It's disappointing to hear not much has improved since I used the station last, which was over a year ago now due to me living in Australia.
I can’t speak for your experiences, of course. However I would say, as a bunch, they’re not rude and unpleasant on a personal basis at all.
Keep in mind it’s generally a fairly thankless job, not well paid, dealing with stressed, abusive, drunk and otherwise downright unpleasant individuals (I doubt I’d get half way through a shift on the barriers without either swearing at someone, or worse

, based on some of the behaviour they have to deal with). No surprise it’s fairly high turnover, many either quit or look for another job internally ASAP.
How much does the typical passenger really interact with gateline staff/dispatchers anyway!?