Steins;Gate 0 [BLJM61303]

I managed to do it! (Sorry for the double post so quickly one after the other)

Here's how:
I swapped out imv008.usm with a copy of intro.usm because I supposed it would play the intro movie instead of the one that wasn't playing. And it worked.
For those are you with the same black screen issue, try this. Then, press L when the video starts playing and it will hopefully skip you into game.
(PS: Before doing that I re-extracted the game and tested with it to make sure everything was in its original state, wouldn't work, I extracted all the modules from the firmware update that comes with the game and decrypted them (the tool only allows to do it one by one so it took a bit of time xD, also wouldn't work))

This game could be moved into Half Playable and possibly Playable, I don't know if changing one small .usm movie file counts as a "hack" to make the game work.
I'll try to play it as far as I can, and make a small video just demonstrating it works.

Screenshot attached.

If that worked then you could probably replace them all with a one frame video of just a black screen; I quickly created a single frame video if you want to try it.