Official firmware is generally available through community-trusted repositories. When downloading, ensure the firmware matches your specific model (GT-I9192) and your region's CSC (Consumer Software Customization) code.

No. Flashing stock firmware does not SIM unlock your phone. It only resets the software. You will need a network unlock code from your carrier.

Partially. You can downgrade from KitKat (4.4.2) to Jelly Bean (4.2.2), but you must also downgrade the bootloader. This can be risky. It is safer to stay on the latest version available for your model.