Hey @Ashemah_Harrison,
Yes, gimbal controllers are really limited regarding their pinout.
And it is true that atmega328 does not have a good support for multiple i2c sensors.
In the new version of the library you change the default bus of the i2c and spi sensor so if your mcu would allow for it you would not need the multiplexor.
Owen will be able to tell you more about the B-G431B-ESC1, he is the big master of it 
I do not even have one.
But what I can tell you is that you will need either a st-link programmer or another stm board with st-link If you have a Nucleo board, that would be enough.
This is a great question. I do not have a good way.
The way I did it is by trying pins. Gimbal controllers do not have too many so it will work reasonably fast.