OEM info hack could be used only when phone is running, connected to pc and via phone you have to ALLOW ROOT PERMISSIONS. That is why you get this ERROR on attached pic.
If there is no possibility to use any update.app (worldwide / serbia 952 or older GB one) and then you can try anything - what about to try that oficial tool for downgrading from ics to gb?
And there is one more possibility. My friend told me, that his SGS3 died, cannot boot it. He bought a HW tool called "JIG" which his SGS3 "resurected" - he was able to boot phone and after that he flashed it...
Here is link - sorry, only in CZ language, use Translator
Maybe it could help - not sure, working on Samsung Devices, maybe others could do that too... And maybe this could be the last thing to try before sending to warranty... Or send it now, before trying