Well, I feel pretty dumb now. I had inserted and removed the wires going to VIN a bunch of times as I was testing different things, and made sure the screws were tight every time. But I never checked on the psu end… the spade connector going to the psu ground was a bit crusty, so I cleaned it up and connected it to another one of the ground connectors and what do you know, it turned on just fine. I was able to move all the steppers, heat up the hotend, and extrude some filament, but I had to leave for work before I could stress the psu anymore.
I guess I ruined the bed mosfet for nothing. Oh well, I think I'll be upgrading to a 110V silicone heater and SSR relay instead anyways, I should just be able to solder a wire to the bed mosfet gate pad and use that (and ground) to control an SSR, correct? Moral of the story, don't be dumb and assume the PSU is working just because it says 12v when you probe it's screw terminals!