[Verizon Samsung Galaxy S III] Droidriven:
First, to truly identify whether the device is I535 or S968C, pull the battery and it will have the true model number on the sticker behind the battery.

If it says I535 but has S968C firmware Then that might be the problem because the modem.img(radio) is different between the two. If it's i535, you need i535 firmware just so that the modem is correct. Don't try to use the I535 modem with the S968C firmware, that will brick the device. Use complete i535 firmware or complete S968C firmware, don't mix elements of one with elements from the other.

Answer #1- it's a carrier issue AND potential a modem difference.

Answer #2 - don't compare PC to android, they are not the same I'm any way and don't use the same rules, same hardware or not. In a general sense android devices with same hardware are somewhat cross compatible in certain elements of firmware but not completely they all have slight differences in stock bootloader, modem and kernel that change the whole ball game.

They have cross compatible system.img but the rest is completely different. There are the very rare few devices with same hardware that can cross flash stock firmware with no issue but 99% of the time it bricks devices.

The major difference between i535 and S968C as far as hardware is that one is 4G(i535) and the other is 3G(S968C has 4G disabled at the hardware level, not software)

The software differences are bootloader, modem and kernel.

The self Destruct chip you mention is called qfuse, it fries the motherboard when software is used that doesn't have the right signature and you can't alter the signature either. Your example of Arch Linux and Ubuntu is not an accurate concept because virtually any PC can use Arch Linux or Ubuntu, this not the case with android. Qfuse can't be defeated either, many, many, many very experienced developers have tried, all they get is a permanent brick.

The way it works is, when the wrong firmware is booted, it fails signature checks and then trips Knox which in turn trips qfuse which fries the motherboard. No way around it so give up on that thought. Yes, it was intentionally designed that way. Knox is not Malware, Samsung put it there from the factory for security reasons. There are apps and softwares that can be used to brick devices but none that do it via Knox, Knox is tripped by attempting to load the wrong firmware at boot (before you get to system where an app can take effect), it's controlled by the bootloader. When it comes to i535, the bootloader is the REAL obstacle, i535 with 4.3 or higher have permanently locked bootloaders that can't be altered or downgraded, in fact, even if you attempt to flash a i535 stock firmware, it will still brick the device if the firmware is older than what is already on the device, for example, if on KK, you can't go back to Jellybean. It's all for security reasons. In the past, devices with locked bootloader could be downgraded to older firmware to allow flashing custom software, the permanently locked bootloader, Knox and qfuse put a stop to all of that, that's why Samsung did it in the first place, they got tired of replacing devices that had been bricked. Yes, basebands and bootloader only want to work with what they are specifically looking for and nothing else.

Answer #3 - custom ROM might help but no guarantees.