Thank you both for your input, I didn't consider bolts and the friction additive (salt or otherwise) is something I never would have come up with. I'll definitely consider those moving forward.
Today I did not get a whole lot done. Fixed the void, put it back with some clumsy chisel work, fixed it again, epoxied the fretboard, found out that it slid (used towels under a soft pine board to distribute clamping force, which obscured vision), peeled it off, scraped and sanded until the epoxy was gone, made better walls for the fretboard to sit against, and just now glued (with titebond) the fretboard again. I'm confident in its position this time, but no clue if this will work out. Whether richlite needs epoxy is debatable from what I've gathered; since I'm out, I don't lose much if wood glue doesn't hold aside from another 20 minutes of sanding.
We'll find out this weekend, I suppose.