So I've been staring at the output_log and found nothing for ages. It was mentioning lost devices and invalid addresses and such, but why? What couldn't the game find? Then, after looking past it a few dozen times, I found it.

desktop: 1920x1080 144Hz; virtual: 4480x1080 at -2560,0

It was registering my Asus VG248QE monitor correctly, then deciding to try and render the game at 4480x1080 which is the cumulative resolution of my setup. I have an LG widescreen as my secondary, which clocks in at 2560x1080. I reckon that would have been fine, but the Asus is 144hz, and the Lg is only 60hz, and I think the different refresh rates were confusing the game, leading to it instead registering the second monitor's width as my refresh rate, but negative.

How do I fix this? I don't think I can, it seems very engine-bound. That said, if anyone gets to this post through Google and needs a quick and simple fix:

Set your secondary monitor to your main monitor in your Display Settings, then launch the game. It will boot perfectly fine. You can then change your main monitor back, but the game will never work fullscreen on that monitor because it wants to vertical letter-box any resolutions less than your widescreens, meaning the right portion of your game will be cut off. Stick it in Windowed mode, 1920x1080 or whatever you want, and then you can play as normal, if not slightly offputting because of the border.