I've added a new picture and added all the code to GitHub, so that anyone may now replicate my results.
I received the PCBs for the optocoupler from China already. And I orderd the parts from Farnell. Unfortunately, I'd picked a part for the 1-gate buffer that they turned out not to have in stock. It's in backorder.
No, I'm not going to wait for 6 months for the part to come in stock again. I have another PCB coming from China already. This one will just break out a MicroUSB connector's pins so that I can solder as thick a ground cable in there as it needs.
EDIT: P.S. The new photo isn't just for vanity's sake. I switched battery holders to be able to move the battery much lower. Despite how it looks, it should be much more stable now.