Hah, yeah, excellent point. Considering the only thing the game is using JavaFX for is hosting WebViews, that's definitely viable.
As far as Nashorn is concerned: last time I was digging through this thing I noticed that the only thing the JavaScript engine is really used for is parsing expressions. Replacing it with an lighter expression parser* might be a good idea.
*I experimented with SpEL a few years ago and that seemed to work fairly well, though I'm not sure how much extra overhead pulling in the Spring library would add. I can't imagine it would be worse than a whole-ass JavaScript library, though.
inb4 you Dockerize Lilith's Throne.