Does this ROM.LOG contain any useful clues about the failure/crash?
Yes. It's an old friend
ASSERT!! FsIoNotify.c Line 451
As it turns out, this not directly related to the reboot function. It happens when something tries to open too many files at once. When there are lots of images on the card, the cameras spends a while doing something with them, which can conflict with CHDK. Having a script auto-start probably makes this more likely to happen. Whether or not it actually happens is sensitive to timing, which varies depending on the SD card and the number of images.
ROM.LOG is a mystery to me but I see "ASSERT!! ... occurred time" is 12 seconds after "rebooting now" time for the failed reboot.
This is a good observation, it tells us this crash was actually associated with the reboot.
The two romlogs you posted a appear to be identical, if you meant to upload two different ones I think there was a mistake. But as long as it's the same assert, there's no need to post more.
I don't have a good workaround at the moment, but I'll think about it.
If you can reproduce it with the build I posted earlier, I will be able to see exactly which file open caused the trouble, which might provide ideas to avoid this specific occurrence.