Burning the sketch with my USBtinyISP never worked, and the Arduino software spouted unfixable errors. This may be due to it being the mac port of version 18. I'll boot ubuntu and install everything there and see if I can make it work.
In exasperation, I thought that maybe, just maybe, plugging everything in and powering it on would make things work. Actually, powering the motherboard might have been good by itself, but we had a far more interesting experience from everything at once.
As the PSU spun up, the Y stepper driver began to fiz from the linear regulator, sparking like a... sparkler. Needless to say, something was wrong.
Now, I know for a fact that the regulators we used are not the ones the schematics called for, but those were on backorder and these were extremely similar. Maybe not enough.
I should really go and dig up the data sheets for them so anyone reading this can learn what to avoid improvising with.
I really hope the $5 stepper chip on the board is not dead.