I made some progress with darkwake=0/2. It seems now, after waiting a minute or two, the laptop does seem to go to sleep. The fans stop whirring, the display/indicator lights turn off. If I try to wake it up, the indicators come on, the fans/HDD starts whirring, the backlight comes on but the screen is completely black. I've Googled, and this seems to be a backlight issue (which is weird because my backlight and brightness controls work fine).
The second issue it could be is a bad EDID, but in your guide you say that's just for Skylake/Kaby Lake systems, and mine is an Ivy Bridge. Is it worth trying?
The third thing would be calling _OFF on wake and _ON on sleep. I didn't do that yet, but I did disable my discrete GPU.
I tried raising the brightness and doing other stuff to fix the black screen, but to no avail. Here's the latest debug files for your convenience.