Mega not showing on Device Manager, Possible fix re-flashing 8u2?

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Hi guys! I've been working for a while now with this mega 2560 R3, and while working with an LED matrix (I made myself) somehow I feeded the mega with 12V (not through the Vreg, but through the pins 5V and GND). It started reseting itself and when I realized what I've done I pulled every cable. No smoke or spark showed up so I'm discarding any burned chips.
Luckily when I connected the arduino with usb the program it was loaded with kept running (the program controlled a 4017 decade counter, btw). The only and major problem I had since then is that my pc (after restarting, also tried with a laptop. Same OS though) will not recognize the arduino in any way, not even the sound it makes when a usb device is plugged in.
Reading for a few days now I realized that the 8u2 might be bricked and I need to re-program it. Found the .hex on github (also on my Arduino folder). I borrowed a  friend's mega ADK which works flawlessly in the same pc-usb port-cable and loaded it with ArduinoISP sketch, connected everything (note that I connected all 6 cables to the ISP header near the 8u2, not to the 2560) and after debugging some sintax errors in the CMD prompt I got the correct commando for AVRdude to load the hex into my arduino. Now this won't work since it says

avrdude.exe: stk500_getsync(): not in sync: resp=0x00
The error LED on my friend's mega ADK flashes while the heartbeat glows.
I used this blog and a few arduino articles to figure all this up but still can't run it.
Am I going the wrong way? Maybe it did burned the 8u2 meaning I have a useless mega.
Any help will be appreciated. And I'm really sorry for such a long explanation but didn't want to leave any important stuff out (I'm sure I did)

NOTE: I just tried the DFU mode to flash the 8u2 with the flip program but I can't get it to show up as a DFU device. Maybe I'm doing it wrong, maybe the 8u2 is actually burned.

Nick Gammon:
You can program via the ICSP header if the 8U2 is no good.

that's what all the explanation is there for. I used a friends mega ADK as ISP to program the 8u2 and the result was the error in the getSync() method on avrdude.

Doesn't the Mega 2560 Rev 3 use 16u2 as USB to serial instead of 8u2?

in that case my mega may not be a rev 3, actually I have no way to confirm it. But I can assure you it has an 8u2 on it (my friend's mega ADK also has an 8u2 and when I plug it in it says mega adk rev3) anyways, it doesn't really matter which rev it is, does it? I know it is an 8u2 and it's not working as it should


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