Aren't both 1st and 3rd person views not using the same models? (Or maybe, does a model file contains both a 1st/3rd person models?)

Speaking as third party observer with a little bit of developer experience and some knowledge of how games/software works, there is little point in changing the current system of createVehicle with regards to initialization.
If you need certain assets to be loaded consistently fast in run-time conditions, then you should prespawn those as "dummy objects" and immediately delete them once you initially enter run-time conditions.
It is justified (and sane) as the developer of the game to leave the existing system as it is, because your case is easily a much more niche use-case than the cases most people use Arma for.

I don't think so, I would expect {mapGridPosition [8500,4700]} to return "085047", not "084046", because 8500m mod 100m = 0m and 8500m / 100m = 85, thus {mapGridPosition [8500,4700]} should return "085047".

Pretty sure I also had a mag still disappear after the update, although it seems to occur much rarer now.

I concur, me and two mates confirm to have this issue as well, both on my custom mission and on Scruffy & NeoArmageddon's co10_Escape dedicated mission.
Since it happens in vanilla Arma 3's inventory handling, without any mission-specific scripts involved, I doubt either me or other mission editors are responsible.
I happen to have it recorded on video, but it's somewhere inside the six hours recorded yesterday, and it seems to occur commonly enough that the video is probably unnecessary.
Personally, as a developer myself, I have a suspect it's somehow related to the two new EventHandlers added regarding magazine (un)loaded, but that is just speculation on my part.