Kernel Caches that aren't cleared properly will do this, the display drivers changes significantly and they're stored in PRAM

That's definitely false. PRAM only contains firmware settings and boot parameters. It isn't big enough to contain drivers.

If the display driver is at fault, grandparent wouldn't even see a mouse cursor.

I don't know anything about 'properly cleaning the kernel caches', but it's clearly not a problem since grandparent was able to continue by rebooting.

PRAM stores a bootarg that boots off the current caches. There's an issue with DMProxy and the display drivers that causes the black screen + cursor issue described. Hardrebooting has the benefit of causing the magic in the EFI to supply a -f (ignore kernelcaches)