This evening I posted an update to the hardware design, resulting on feedback I have received over the past week. Major changes include adding a FET driver to the Field circuit replacing the original NPN - Pull Up. By doing so, I expect on/off transitions in the FETs to be much quicker during PWM control of the Field (Esp during the Turn OFF transition where previously the 4.7K pull-up would be the limiting factor).
I also corrected a major design error around the I2C bus extension power supply. The original simple Resistor / Zener did not take into account loads placed by the pull-up resistors and just would not have worked. A new 9v LDO regulator takes its place.
And finally I have decided to be a Lemming and stick with the High Side field control. I still can not find justification of why but am so bothered as to how almost universally every example of alternator control devices I can find drive the High Side. There MUST be a reason, and though a low-side driver does make more sense from a FET driver standpoint, I can not shake the obvious direction the industry has taken - even I have have no clue why.
So, high side it is.
Going to let this sit for a bit longer as I posted the project to another website this evening. After a week will look to get the PCBs and parts ordered. Want them to be at our mail-drop for our trip in December.
Then the REAL fun begins!