So, I found a rather convoluted workaround. I edited my save on my PC as usual. I wrote a Python script that copies byte by byte from the edited file to the working file. But the key point here is that I run that Python script on my phone using PyDroid3. This proves it's metadata that's the problem and not the save file or the changes PKHex is making to it.
Another interesting note is that when I first tried to do this, I ran the script in the directory that I had explicitly set as the directory for Drastic in the Drastic General settings. When I first tried after doing it there, Drastic gave me new errors about the directory not being usable and I obviously couldn't launch then. So, I made a new dir and copied all the relevant stuff to it, and after pointing Drastic to that new dir, it worked--my beautiful Golem was alive on my phone. But I think this just points to more Android file permission/metadata issues that I don't really understand. I think PyDroid must have changed the permissions/metadata of one of the files or the parent dir when it ran. But now I just copy post-script-copied file to the newly set Drastic dir and it works...
Convoluted but functional, the motto of all software everywhere.
The script is roughly as follows:
import os
writePath = '4780 - Pokemon HeartGold (U)(Xenophobia).dsv'
readPath = 'Edited-NotGrav-4780 - Pokemon HeartGold (U)(Xenophobia).dsv'
with open(writePath, 'wb+') as writeFile:
with open(readPath, 'rb+') as readFile:
writeFile.write(readFile.read())