Well, I figured out my first problem - my testmode folder was inside another testmode folder! I must have extracted it wrong at first.
Even after I fixed that, though, I had problems. I went through something like three different testmode folders downloaded from various links on this site. I tried reformatting my SD card too. But no matter what I do, I can't get my stereo to boot into testmode again. What happens is it shows me the splash screen, and then the screen turns off and back on again. Repeat forever until I remove my key or pull the SD card.
Is there a fix for this? I can't think of what else to try. Without being able to get into testmode, I can't seem to do anything. So for now, I'm stuck with a unit that cannot remember my settings and can't take any further software updates.
The good news is it still works fine with the default software and I did manage to update my maps to 2009.6 before it stopped letting me use testmode. It still remembers my radio presets and my favorite addresses. So it's not totally useless, it's just frustrating that it doesn't seem to want to enter testmode.