Please give me the numbers on the board. We have been getting ready to release a new board version, but I didn't think it was time to do that yet. You may have gotten an early one.
We have to update the documentation before releasing this!
JP6 and JP7 have been removed on the new board. You can cut and solder if you want to power I2C mux segments other than what you are feeding the board through J1.
No, you can't power the board with JP8. You need to feed it through J1 or JP1. BUT, you can't power the board from 5.0V if you are feeding it 3.3V signals from the Raspberry Pi. If you feed 5V into the board, then the pull-ups on SDA and SCL will be pulled up to 5.0V which may very well fry your Raspberry Pi inputs. If you are feeding the board with 3.3V GPIO lines, then you need to power it to 3.3V.