Unlocking the bootloader will void your warranty, that's for sure. Flashing another country's stock firmware doesn't need the bootloader to be unlocked, but it still constitutes modifying the supplied phone software and you may be rejected if the service centre finds that it is not the Singapore firmware. But of course, you can always try to flash the phone back to Singapore firmware before sending it in for repair / servicing.
Any country's firmware will work, as long as the model is the same. Our dual-sim XZ model is F8332. For me, I flashed to the India firmware, only because I could easily find the 6.0.1 firmware online and it can be upgraded to Nougat. But the India firmware did come with 3 games that cannot be uninstalled and can only be disabled.