Thanks for such a hastle free upgrading process, i upgraded to v4.3 with ease, this is what i did while on 4.1.2 custom rom, downloaded mj5 bottloader+modem+recovery (final.md5.tar), downloaded twrp open recovery (openrecovery.img.tar file), rebooted phone in download mode (i wanna add here don't reboot to download mode using current 4-way reboot menu, only do that by turning phone off then press volume-+home+power and then press volume up to continue) and flashed final.tar.md5 as pda, when it finished installing, it rebooted to stock recovery, take out the battery and put it back in and enter download mode again, install openrecovery as pda and after finished installation, phone will automatically boot to twrp recovery, now perform full wipe and flash custom 4.3 rom, after flashing is done, dont reboot, install dr.ketan multi tool advance v6 and root + knox free using aroma option, and after doing this reboot your device, you'll have fully functional 4.3 rom with no wifi or random reboots etc issues.
Also i don't think flashing knox free firmware and at the same time worrying about warranty bit is justifying..you are doing nothing than wasting your time, samsung will not accept it if you have firmware without knox, nor do they will accept patched mj5 bootloaders, so why should we worry about it? Flash this patched bootloader and flash a custom rom, if you want to sale it , just flash your region's firmware.
Mobile odin will help you with the knox free firmware of op, but i am sure many of us dont belong to the region that has this firmware, so definitely this firmware will still not help you with warranty claim or reselling your device, if you want to keep your device and use it, flash this firmware without worry.