Flashing Stock Android ROM from another country
Yes, it will void the warranty; 
Hard to say, some Samsung firmwares are available "officially", some are leaked, and some are not available at all. XDA forums for you device are the best place to look; 
KNOX counter is a way of checking if you phone is or has been modified, in theory flashing a stock ROM will NOT trip the Knox counter; 
Probably; 
If you are stock, you are stock, the source of the image isn't relevant since the image will be digitally signed.... 


The real issue here is the hardware/radio differences that cannot be edited or flashed, it likely will not function right, that information is protected and tied to the device, flashing a ROM does not change bands or network connectivity of the hardware, but it does change the radio firmware which can cause a mismatch of hardware capability to radio firmware preventing connectivity.