Neither modifying config.txt or (just to see what would happen) removing the ground from the ground+video pair on the Pi seemed to help. Still just a powered on screen with no image.

I appreciate the help, I may just get another screen.

-Just tried formatting the SD card and reinstalling Retropie. Tested OK with HDMI but no dice with the screen. -Multimeter tested OK as well with the connections, including power+ground from where the regulator was all the way to the Pi and video+ground. Decided to switch to AV2 anyway to see if tha...

Thank you for taking the time to check!

Cable looks good to me (a small cosmetic blemish) and seems to be seated well.

New to this, so please forgive the terrible soldering (though that may be the problem itself): I'm unable to get a picture to my Adafruit 3.5" display used in the original guide. I've wired it as far as I can tell to run off 5v and I'm trying to test it with my Pi Zero. I can see the display is back...