First Wire Up and Motor Spin
1. Spin all the motors up from Python.
2. Power Raspberry Pi from the same battery via UBEC.
I had to do a little soldering to connect UBEC to my power distributor board, but everything else connected nicely and behaved as expected. I was able to spin motors up and down manually from Python over the ssh connection. UBEC was able to power the RPi without issues (in the image UBEC is unplugged and Pi is connected to an external power supply).
So far, things are looking promising.