I ran boot-repair, so here's the link to the report: http://paste.ubuntu.com/p/fTmkr8mQwq/
I'm posting this from the live session.
So, when I got done re-installing Lubuntu with properly set partitions for UEFI/EFI mode, I ran boot-repair, then rebooted. When I tried to go into it, it showed the loading graphic for a bit, then black screen (with blinking cursor), and then a little later this error (not exactly the same as in the number preceding it ['cause I saw this before the whole process when I set "quiet splash" to "splash", but otherwise the same).
[85.063480] snd_hda_intel 0000:00:03.0: HSW/BDW HD-Audio HDMI/DP requires binding with gfx driver
Hand-typed it but I wrote it down so it should be spot on.
I have a feeling that this has to do with not having the proprietary Nvidia drivers.
Nvidia 820M if you're asking.
I'll try to go into recovery mode and *try to replace the Nouveau drivers with the official Nvidia drivers, and see how that turns out.