KerneelsM:
I am also considering RS485. Do you have any part numbers for the isolated drivers?
I'm using these RS485 modules when necessary (in my usecase I needed isolated communication with MPPT solar charge controllers)
Did you consider one Arduino (Nano) with one A/D input (for the whole lot),
and small relays with "flying capacitors" for each cell?
That would also have given you 14x the resolution.
Leo..