Just an update: I fixed my problem by brute force. I say "brute force" because I don't know what I did to fix it. I still suspect I fudged up one of the yml or properties files when I set up PlotSquared for the first time a few weeks ago. Anyway, today I saved my plot with WorldEdit, wiped the server clean, set it up again from scratch, and then used World Edit to import my plot. Dropped entities stay put and redstone works correctly! woot!