I’ve not read the whole thread.
To my mind there’s a clear relationship between accidents like this, and the European habit of carrying a loaded rifle when stalking and relying on the safety, as loading a rifle when presented with a shot is regarded as too slow and/or too noisy. If you make a mistake and forget to check the rifle is clear before stowing it, the likelihood of there being a chambered round is massively reduced if you aren’t in the habit of loading one when you don’t need it.
There are YouTube videos of European hunters getting out of the car, removing the rifle from it’s case, chambering a round and slinging it over their shoulder and walking off. That’s a massive no-no in this part of the world. Here, if you accidentally shoot someone in circumstances like this, it means you’ve broken the rules of the Firearms Code, which you promised you wouldn’t do when you signed the firearms licence forms. So if you do you’re in deep shite.
From the account it seems that the client did not properly unload his rifle before putting it in the Landrover. And neither did the guide check that it was clear before it was put in the vehicle. This has got nothing to do with our "European habits", it sounds like a violation of every rule in the book, or common sense.
As to what then happened, well that will perhaps be the subject of an enquiry, by the Police, and the insurers. Just to observe that well maintained rifles generally don't go bang unless the safety catch has been released, and the trigger pulled. Even then they don't usually shoot people unless the muzzle is pointed at them.
The "European Habit" as you call it, is the way that I was taught. That once we enter our hunting area we load the rifle and apply the safety. Then always assume that our, and others' rifles, are loaded and ready to fire and behave accordingly. Unloading when necessary e.g. to traverse obstacles, climb into a high set etc. and before returning to our vehicle, not waiting until we get back to it.
This is what we are taught at DSC1, for lowland stalking. For example, read the DSC1 syllabus on rifle safety. I dug out an old copy, 2012, but AFAIK this is still what we are taught, and examined on. See question 2.
For Scottish hill stalking, where the rifle is typically carried in a slip until the final approach, the questions start at 20, and may be more in accordance with how things are done in NZ.
See also
Safety | Best Practice Guidance which is more comprehensive. Nevertheless the practice of stalking (i.e. moving around) whilst carrying the rifle in "stage 4", loaded and safety applied, is normal.
•
Only disengage the safety catch when just about to take the shot. (Change Stage from 4 to 5*).
• Apply the safety catch as soon as the opportunity for a shot passes.
• If stalking with a rifle with a chambered round (Stage 4*), keep the safety catch applied, and the rifle slung securely over one shoulder.
• If actively stalking with a rifle in Stage 4, ensure it is kept pointing at the ground or into the sky at all times.
• If shooting from a vehicle, a round should only be chambered (Stage 4 or 5*) just before a shot is being taken when the muzzle is located outside the vehicle.
I had been club shooting for at least five years before I took my first deer, so safety, from a target shooting perspective, had been drummed into me, the rifle and magazine only ever loaded when at the firing point when the RCO commanded, then unloaded, bolt out, and checked clear firstly by me, then by the RCO, before allowed to leave the firing point. Finger outside trigger guard at all times, except when just about to take the shot. That drill still serves me well when going for guided stalks, the difference being that the firing point is now the hunting ground, and I may roam upon it with the rifle loaded.
As for fully checking clear, on a target range in good light this can be done visually. Nevertheless it I always, as a final check, use my little finger to probe the chamber to make quite sure that there is not a round still inside it. It only needs say a fault with the extractor, or a short stroke of the bolt, for the rifle to seem to be unloaded, magazine out or floorplate released, bolt then re-inserted, for the rifle to still be ready to fire. This (finger test) is IMO the only way to properly check clear, I do it every time.
I also count my rounds in, and out again. It's not difficult, particularly if you keep them in a bullet wallet. If you have used one or two shots, then you should have one or two empty places in the wallet (or empty brass). When using a lever action, when removing the bolt is not possible, I rack out the magazine into my hat. There should be six rounds in there (minus any fired).
There are too many subtly different types of bolt, and other action, rifles, for even the most experienced to be familiar with every type, or swap between them with complete confidence. Integral magazine with floorplate, drop magazine, controlled vs push feed, flag safety, lever safety, safety inside the triggerguard

two positions or three, does the bolt lock closed on safe meaning you can only load with the safety off, most fire with the safety in the forward position but some others are forward for safe, rear to fire (I really don't like these). Does the safety block the firing pin, or does it simply block the trigger, leaving the pin held only by the sear. Has the trigger been fiddled with to the detriment of sear engagement, or replaced altogether. Has the rifle been thoroughly bump tested after such an adjustment. Even if it was originally, has this been verified recently. Or is it a decocker type of safety, very safe when decocked but no different when cocked, which may take considerable force and be clicky. Yes sometimes you can overcome the click by pulling the trigger whilst cocking then release to hold the pin in the cocked position. Hardly good practise. Has it a set trigger, if so do you know how to unset it safely if you don't take the shot. Do you make use of the cocking piece projection to ascertain whether the firing pin is cocked or not (assuming it has one). And so on.
If you chose to carry in stage 3 (magazine loaded but chamber empty), how good are you at unloading the chamber then pushing the round back into the magazine then closing the bolt whilst keeping the round pushed back into the mag. if you don't take the shot ? Assuming an integral mag./floorplate arrangement. If that is even possible with many types of drop magazine. Do you instead take the mag. out put the cartridge back into it, close the bolt, maybe even dry-fire it to decock, then put the magazine back in again ? Lots of ways to get some of that wrong if e.g. tired, low light, cold fingers, unfamiliar rifle etc.