Frequency is basically zero, so common mode rejection should not be a problem even with cheap op-amps. Remember that it's not necessary to sense voltage down to the mV, what you really need it to be able to tell when cells are close to their max, which can easily be done with "only" 10mV of resolution. Do what you please, but I would not remove a few pennies worth of resistors and cheap op-amps in favor of a substantially more expensive amp just to reduce part count.
Also, I would suggest calculating the current and voltage in each node of your op-amp circuits. I haven't done the math myself, but I have a feeling you're drawing more current through the 20k resistors than you might like.