Cool. Thanks for the advice, and clearly explained. Just an update:

I decided to go ahead and replace the sand in the filter. I figured I didn't want to use a lot of bleach just because I had a bunch of gunk in my sand. Now I should be ready to add CYA and top-up levels of bleach. I'm adding enough CYA in a sock to bring me up to 35ppm tonight, then enough bleach to bring me to 2ppm tomorrow night. I don't have a good test kit yet, but I figure estimating won't do any harm for now. (I'm running on a really tight budget right now, so it'll be a few days before I can get one of the kits you recommended.) When I get the kit, I'll post the results for the pool water and for the well water. For now, is there anything else I should know, any pitfalls I'm stepping in?

Peter