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)
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.