Raspberry Pi 4b: LightDM won't start because Xorg Can't See Attached Display

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?