Hello ! Sorry, I unfortunately have no experience with Proton at all... but if you can give me details, I will try to see if I can help.
The game doesn't really use the default RTP but custom assets as well as custom scripts. Those are setup properly as far as I know but if it required an additional setting for Proton, do tell. But.. if the game must only use default RTP to run on Proton, then, yeah, that's a problem here...
However, uou said it's not even launching though. Usually a ressource problem will still launch the game while displaying an error on screen or just waiting for the ressource in a black screen. So, it might be something else ?