Oh yes, hello!

I think it is worth making a prototype of this with some old SLA batteries or something with a load and measure the voltage at each take off to see what happens. You could put in a inline diode on the drive circuit to prevent a possible problem there but I'd possibly try wiring the battries up in series in one go and then having a take off for the drive with something to reduce the voltage inline to the drive like a potentiometer or something. That way, you'd only need the one fuse for all the batteries, which simplifies the wiring.

As far as cell protection is concerned, I am guessing the balance plugs connected to the Arduino to get a reading for each cell would work with a inline relay between the 4S batteries and the 5S battery to cut the circuit if it was needed but it feels like a lot of parts to go wrong.