Dang. I actually fixed a thing.
I loosened both copper terminals, which were cranked on there hard, which tilted the contacts, and retightened them to a lesser torque, while ensuring the contacts stayed level. Popped it on the rig and it fired up right away.
If you look at the contacts, you'll see the output has uniform arcing while the input has 1 or 2 very slight pits.
Instead of returning my core starter to Toyota, I am motivated to rebuild it with me new found knowledge and put it on Zeke, as he has an el cheapo direct drive starter right now.