Multiplexing battery voltages with high precision
It is not OK to think each battery is floating, when your concept of sharing some 0V between all of them due to stray capacitance.

They must be have some common mode 0V even when you have a MUX.  

Therefore if there the extras system impedance between these cells is always high and &lt;100pF you can use a Two Pole MUX with an INA using a twisted balanced wire pair that has more shield capacitance to 0V analog than the capacitance between cells.  You still need to consider ESD and current limit protection on all inputs to protect from cap discharge or crosstalk capacitance of your
 MUX .  and settling time.

On long cables with this criteria where common mode input impedance is low and differential is high , you can still get noise on SAR type ADC’s so a cap snubber on inputs with ADC trigger delays can suppress noise to &lt;1mV. 

Whereas integration type ADC’s automatically remove noise during slower conversion.