So this is a strange issue.
After switching kernels, LightDM refuses to start, on grounds that there’s not a display plugged in.
So far I’ve tried two different micro-HDMI to regular HDMI adapters with an HDMI dummy plug (I normally run headless, but do this to trick X into starting), with no change. I grabbed a 10 foot HDMI cable and hooked the Pi up to my TV, got to watch the bootloader do it’s l33t hacker/matrix-style scrolling, and ended up at a black screen with no login prompt.
I’m pretty convinced this is a software issue. How do I either uninstall LightDM totally and reinstall it, to get it back to defaults, or in the alternative reset it without reinstalling?