Good debugging tips from all these folks Tim, but despite being relatively experienced IT guy up to a point, I'm also a weird combo of goal-oriented and paranoid. LOL.

If it was me I would have stopped messing with the main drive and done what E.J. first suggested -- put in a new system disk and done a clean install of the OS. Then worked at transferring over info I may need from the old hosed disk.
Massive corruption, failed chkdsk, disk full, aborted installs of multiple OS releases... this is a recipe for a total meltdown and a ton of wasted time.

Continuing down the attempted troubleshooting route in this situation feels like a bit of target fixation to me. My advice is cut your losses and bail... do a clean install on a new disk and recover your apps & data from there as necessary. A clean install will probably benefit you for other reasons anyway, giving you a nice fresh base again.