First of all, thanks for staying with me all through this.
I've recorded a log with Procmon consisting of opening Kodi > opening a video (with MPV) > searching for a subtitle (with one script) > searching for a subtitle (with another). The result being that neither script searches for subtitles, and that instead both immediately jump to a "Subtitle search failed" message - no Python crash though (I believe this would happen either because of another version of Kodi, or another version of Python). The log was huge, so I narrowed it down to the first appearance of Python, until the last:
https://mega.nz/#!HAcUHYhK!bVULUKRuWcumb...YvuqQqXqwM
It's somewhat of a relief if this is actually OS-dependent, as I hope to jump out of Windows in the future - but still, I'll probably stay here for a while, and as of now it's a big inconvenient for my setup (that is nevertheless not as inconvenient as having to abandon MPV). I don't have any computers running a different OS to test this out, so if you, or anybody else reading this, can try it themselves, it'd be a big help. The process from beginning to end, to get this running, is:
1. Download MPV:
https://mpv.io/installation/
2. Set up Kodi to launch videos with MPV:
http://kodi.wiki/view/External_players
3. Download Python, if not already present on the OS
4. On your OS's console, run "pip install subliminal"
5. Create a "scripts" folder, on the same location were you have mpv.exe
6. Save either of these scripts, as "autosub.lua"/"subit.lua", on the "scripts" folder:
https://gist.github.com/selsta/ce3fb37e775dbd15c698 /
https://github.com/wiiaboo/mpv-scripts/b.../subit.lua
7. Change the "etc/etc/etc/subliminal.exe" path were specified on the script, to the one were you have "subliminal.exe"
8. Load up a video from Kodi, press "b" or "d" depending on the script (note that these aren't the same as pressing "B" or "D") to search for subtitles
9. See it fail immediately
10. Load up a video on MPV, without Kodi, and see it at least try to find the subtitle
Hope this can narrow down the error!