I love how the guy knows he's shorting out the breaker, yet does it AGAIN. Hello, asshole? YOU HAVE A ****ING SHORT SOMEWHERE, QUIT ****ING WITH IT AND CALL AN ELECTRICIAN!!!
I worked with a guy in my previous job who was not an electrician (he was a comm tech) who tried to do live work inside a 240 panel.
He's dead now. Let the qualified licensed contractor do it. Yes, it will cost you some money. Yes, the job will almost certainly be done right, and will certainly be done better than you could do it. NO, you are not equipped to fix the problem you may or may not have found.
Like I said, someone, somewhere, did some shitty work in that house, and needs their asses kicked.
Oh, and DUmmies? Very little wiring below the 20 amp level is 12-gauge. Most is 14, or even 16 in your small appliances. In my old house, the AC was 10 gauge, and that was even for a 40-amp service. A lot of it depends on the type on insulation used on the wiring as well. Even on a 200-amp service, the run from the pole to my meter is 2/0, 3-wire.
Edit--checked the panel--it's 2/0. Sheesh.