1. I'm wondering if/how/where there is a utility/program/etc. that I can use to browse older addon code to check for outdated commands or ui items.

    I'd like to resurrect some long outdated addons, for instance Fizzwidget's reagent cost.

    Any ideas?

    No, not really. Run the code, and when stuff errors out, you can fix it. A tool like that would take a bit too much effort to really be worth it when so much hand-tuning is necessary anyway.