Well, my poor truck has been at the audio/alarm/electronics shop for 2.5 weeks so far. I made a surprise visit today and saw it on the floor, along with a bunch of other vehicles. The shop is the go-to place in the area, and they are really swamped. The owner said they've put about 12 hours into it so far. He said it's been sitting on the shop floor for 2.5 weeks because it's a really difficult job, and he wants to have the right people focusing on it and take his time. But he showed me what they did so far. Basically they disassembled the entire lower dash area and pulled out all the obstructions in the engine bay interfering with firewall access. After doing that and getting good access to the firewall, they found a piece of metal stuck into the big harness that leads to the SC ECU. Apparently, when I was fishing wires through the hole, a piece of something broke off and got lodged in there. It's likely that's how the 4 wires got damaged.
The next step, he said, is to pull out the harness sufficiently to inspect all the wires for damage, find and repair the four I broke, and fix any other damage that might need attention. He still hasn't decided on the best approach to doing this. He is also leaning toward a conclusion that the firewall was also overpacked. He will likely expand the firewall and rewire some things in the process.
Looks like this is going to be very labor-intensive and costly job. Folks, take care of your firewalls! Don't make my mistake.