I just found the problem, and I feel stupid now.
I checked Task Manager, and multiple instances of Skyrim were running. Which makes me think that it didn't close out properly after its first test run, and then I failed to notice that the .esp didn't save the first time and kept going. I first noticed something was up when I tried to generate the grass cache and the game wouldn't fully load, which just led to MO helpfully trying to restart the game for me every time I tried to close out of it, and it continuing to run in the background despite not being on my taskbar. Once I took care of that, the clean version of the .esp saved successfully, and grass cache generation is underway.
It is physically impossible for me to facepalm hard enough right now.
Thanks again for helping to troubleshoot.