I know very little about aviation but this seems like a pretty foolish and avoidable incident. I would have thought there would be a procedure for how many people can leave or hang from the back of the plane as it would very obviously affect how it flies. I can’t imagine that sort of manoeuvre is good for the airframe?
You are right that there is a clear limit to the number of people allowed on the outside step set by the DZ. In the original video you can count the heads on climb-out and these jumpers are exceeding every guideline that I've been subject to. My formations adhere to five outside and one crouched, max - but do note that several bodies may follow the exit with as little delay as possible. We try to exit as a unit. I'm only a coach with ~500 jumps, by no means an authority.
This model plane in particular is less forgiving and the airspeed is high on jump run. As skydivers we are cautioned about the exits and take extra care to maintain a forward CoG. I have experience with aerodynamic modelling and private pilot training but won't speculate other than to say that structural integrity is not an issue. It's more like capsizing a boat than overloading a trailer hitch.
This seems like a weakness of a king air as a jump plane. The pilot has to throttle down the left engine so jumpers aren’t thrown into the tail on exit by the prop wash. I’m happy with our single-engine super caravan with its wide door so not so many people have to be outside to do big groups like this.
I always find it interesting how we underestimate the expanse of 3D space depending on the view: jumpers are exactly between the airplane and camera, which is not surprising since the cameraperson and jumpers came from the direction of the airplane, with the camera going out first.
From another angle (eg satellite view from above or a sideways view), I am sure the distance between jumpers and the plane would be more obvious. Still risky (you wouldn't plan on passing that close), but not as much as it seems.
The question more is what made sure that the airplane didn't re-contact the skydivers. When an airplane after the drop flies on straight and steady you know that they wont hit the skydivers because the skydivers have no propulsion and relatively lot of drag. The skydivers will lose horizontal speed quickly and gain vertical speed downwards even quicker. This basically guarantees that the separation between the squishy humans and the airplane will be maintained.
With the spinning airplane staying close to the divers who knows. Maybe the pilot is an ACE and they kept a mental model of the parachutist's position relative to theirs at all times. Maybe what appears to us as uncontrolled tumbling is anything but. Or maybe it was just up to "luck".
The rest of the dynamics fall out from that: during the dive, the engine on the divers' side is already off, and once the stall happens, the pilot cuts off both engines (to avoid a spinning engine hitting anyone), dives to gain speed.
Being inside the plane as a jumper during the recovery was probably the least pleasant -- normally one doesn't have a seatbelt in that scenario. I think at 0:54 one more person falls out the open jump door?
I like to turn question like this on its head: how would you make an airplane hit those skydivers on the way down? If the answer is: "very fricking hard", I think the "luck" plays a small role.
Basically, if they were hit, they would have instead been "very unlucky". :)