Oh okay, let me post what I figured out:
The cutscenes are, clearly enough, inside "movie.bin", but we can't decompress them the way we got the other files. Luckily enough, there's a couple programs that can extract them, like MultiExtractor:
Or Nova Extractor, which kinda shows the same results:
MultiExtractor has a paid version that lets you patch in a movie (I guess basically replacing one file with whatever you tell it to?), and it kind of works indeed, since I managed to duplicate cutscene 2 into 1 as well, so that whichever you played in-game, you'd get cutscene 2 anyways (I should probably have taken a nifty vid here, but eh, it's easy enough to do again). Also, this patching only works if the file is equal or lower in MB size.
However, that's basically as far as my successes went. I tried to add some sample .ASS sub to the first vid and save it again as MPG (also tried converting it to MP4, subbing it the way I usually do, then converting it back to MPG), but patching it either failed to play anything, or the movie played past the halfmark without subs (maybe because it didn't understand my patched vid and it skipped to the remaining part of the original vid?)
So then I looked at the "movie.bin" with wxMEdit to see if I could at least identify what kind of encrypting it was using, and sure enough, it's a "PSMF" thing:
Buuuut yeah, that was opening a whole can of worms. Apparently PMF is a format the PSP famously uses to play vids, with sound encoded with an Atrac3 whatever (which explains why my conversions never had sound), but tools to extract/convert/play them have outdated links by now. Some people say that the MPGs we got are the same as PMFs so only a renaming is needed (
https://www.animemusicvideos.org/forum/viewtopic.php?t=82457)
And there
are some people who apparently have subbed ingame vids (so some hope there):
https://www.romhacking.net/forum/index.php?topic=24307.0, but their process of splitting video and audio doesn't seem to work with our MPGs, renaming or not...
