After downgrading Apache, installing Xdebug for PHP, clearing the in-database Drupal cache for each test site repeatedly, and dancing in the rain with a dead chicken in nothing but my underwear, I've finally gotten the test sites on the laptop up and running.

Most (if not all) of what I actually did (which involves all of the above except the dancing) was pretty much just voodoo debugging. I wish I understood how it all worked, and I wish that Drupal was instrumented in such a way that one could reliably trace how pages get generated. I was daydreaming about writing a PHP module for debugging, that would track state changes with each function call. Actually, if I could use the Visual Studio debugger for PHP debugging, that would win.