ObsessedWithGTA5

Hi there, I'll answer your questions for you as I have a fair amount of experience with these.
GTA V will "handle" any vehicle that has been left in a place where it is not supposed to be. It often does this either by straight up despawning it or it will sometimes assign a random ped to steal the vehicle and drive it out of existence.
The game draw distance is not entirely accurate, in fact only about half of what you see is actually an entity/exists. The other half is merely a placeholder for when you get close enough. The issue with running lots of scripts is the memory gets allocated to the scripts and that means the cars get despawned a lot faster than they should in theory. but equally this isn't always the case and the game can just delete the cars anyway.
This problem caused issues for police patrols, so like you said MoreControlV saves the player's last vehicle. It makes it persistent (forces it to be saved in memory and not be despawned. You can save a car in Los Santos and fly a helicopter to Paleto Bay and back and the car will still be where you left it PROVIDING you didn't exit the helicopter.
As for the vehicle locking, an override mechanism exists for if the locking system fails, or your script crashes etc. Use LShift, LCtrl and the unlock key (default L) will force unlock the nearest vehicle to the player.
Lastly as super stated, read the configuration file with MCV to see how the locking works, there are a couple of options there for automatic unlocking etc.