1. There is a possibility that I've exposed the 5v and GND pins for a long time since I used them as a power source while I was soldering a PCB board
This sounds highly suspicious, even though I don't completely understand what is being said.
Well while I was soldering a PCB board I think that I somehow linked + and - together but I haven't realized that. In fact, you could take a look at a project I've done. http://youtu.be/1IwfZIiVXiA
It's a PCB with a Attiny85 on it and I use the Arduino as a power source. Note how messy the soldering part is at the back of the PCB board. On the video it worked perfectly but as I began to develop the board further, I somehow messed up with it and probably burnt my Arduino.
Yes, assuming the only problem is a corrupted bootloader or ATmega16u2 (the usb to serial chip). I'd start with the USB to serial chip, since your computer isn't talking to that one anymore.
Yeah, the ATmega16u2, I think that that the exact problem. I've heard about the USB to serial chip converter but I costs the same amount as buying a new Arduino. I've also heard that you could somehow use the MISO, MOSI and SCK port to program an Arduino using another Arduino. Is this possible?
Thanks for your response my friend!