I've spent another 2.5 hours in easyeda and the datasheet and I think I've got a pinout that will do what I want it to. I've used USCI1_CLK and USCI1_DAT0 as Clock and Data respectively, which is just an educated guess since the datasheet doesn't specify what pins are actually used for the I2C configuration. Additionally, the "ICE" pins are the chips debug pins, so it's probably a good idea to add those in.

I've also started to rip 'n' tear out the traces I've made so that I could move the MCU up high enough to comfortably fit vias on the bottom side.

This is the reason why Pin11 is NC. There's not enough space for a via. Additionally, Pin15 is not a BPWM (Basic PWM, but it seems like an enhanced version of "PWM" found on the M031 when quickly skimming over the datasheet) pin, so I couldn't use that for anything. Conviniently, I still have enough prins to connect everything I need.

I still find it a bit suprising that, if I'm understanding the datasheet correctly, there isn't really all that much stuff I need to add an entire microcontroller to the circuit. I did a bit of looking, and the evaluation board for the 32K flash version looks pretty sparce:It really looks like the chip and 3 capacitors is all that's needed for most operations.

I can't find any schematic though. However, I can see that the 2 USB data lines seem to be length matched.

Still though, it's strange to think that a 288 page datasheet actually seems a bit lacking in detail.