Everything is now sorted out My friend walked me through the entire rear-wheel-removal-replacement-brake-issue process again and I finally realized what I was doing incorrectly with the brake cable. I was, apparently, trying to "snap" it in/out of the tab rather than just lifting/moving it. I found it also helps to stand with one knee against one of the calipers to hold it against the rim.

I DID swing my bike by the LBS for a quick safety check considering how much work I did with the brake cable adjustment - really more for peace of mind than anything. It was fine, and no charge