Galaxy S3 won't Power On / Charge / Read from USB

I woke up few days before while I was keeping my Galaxy S3 (GT-I9300) in the charger and found out no electricity, however, I found out that the green LED is blinking too fast. I pushed the power button to get it unlocked but it didn't respond. I had to hold the power button to force it restarting (as usual in such cases) after unplugging it from the charger (for any expected power return back to source), however, the phone turned dead. It is no more powering on, I replaced the battery with another one, I used the charger when power is back, another charger and connected the USB to PC and nothing happened! I left the battery off the phone for several hours but it didn't work, though. The phone is now hard bricked but would this case be a system board failure (high voltage) or just the bootloader became malfunctioning? Since the phone is out of warranty, I opened up the case, removed the battery and opened it and removed the system board carefully (I'm a Computer Engineer and took my precautions). Then I used a multimeter continuity check to make sure there are any charges. I applied it to the power button and the multimeter read 1 with high sound while any other part to read very high value with no sound. I had to send it to the official Samsung repair shop for checkup and they told me it might be a system board or software issue (they didn't explain but I understand something regarding the bootloader/EFS...etc) and a system board would cost about 330 USD!! I should know what happened by tomorrow and decided I won't replace the board at all in such a price but I'm assuming it's a hard-brick. 1- The LED was blinking before I had to force hold power button to get the phone restarted, hence I think it didn't get high voltage from power source. 2- Would force holding power button to get the phone restarted to cause a power damage to the motherboard? 3- Would force holding power button to get the bootloader malfunction? 4- The multimeter's continuity test I performed has any sign/meaning? 5- Since I have a custom ROM with root (I did installed tens of custom ROMs before with tens of root/unroot) and I didn't suffer a problem, my local stock Samsung firmware yet before MG4, would this be a problem if it's a bootloader issue for them to use JTAG or whatever to copy bootloader information? (Modems after MG4 has another store location for EFS rather than earlier once) 6- Would USB JIG would just solve it? Any tips / feedback would be most appreciated.