[harlequin2]

First off, you can't use a fet as a load without some form of stabilisation - simply applying some voltage to the gate results in a completely unknown and quite unstable current flowing. "Calibration" as you suggested does not work, it is simply too unstable to be even thought about.

I did experiment with using photo-emitters and receivers in lieu of wires and optocouplers, but have not been able to obtain reliable results and I wanted a working bms in my car, so that idea has been temporarily shelved. One of these days I will re-visit it.
Item 3 - I don't understand how the addressing might work.

No rivets used on the pcbs, I think you are looking at some solder on the plate dthrough holes.
The internal oscillator in the PIC is +- 2% from 0 to 60 deg C and only +- 5 % down to -40 deg, so its much better than the AVR R/C oscillator. It is stable enough for the RS232 to work over that temp range.