I waited over an hour for TurboTax to get over its hang, but nothing happened (not surprisingly, since Activity Monitor showed no CPU activity). So I went ahead and mucked about with the forms, and then went back to EasyStep mode. That put me back on the step where it had asked me whether I'd like to file an extension. After clicking through several screens, I ended up back with the original questions (the ones whose answers I'd edited manually on the form).
So, the upshot is: Removing state form data appears to be impossible (at least for DC) on the Mac version of TurboTax Premier. And (for DC, at least) you can't edit any previously-entered data until you completely go through all of the questions about filing an extension. You can't step backwards out of it, and you get stuck back in the same place if you restart TurboTax. In fact, for all I know, the issue with removing state form data only happens if you're stuck in that sequence of questions about extensions (I could go and do some experimentation to figure out whether that's true, but I don't feel like doing Intuit's QA work for them).
Intuit definitely needs to do better QA on their State products. I've had no problems with my federal taxes, but the state product seems to be very hastily thrown together.
