WB Last?
 

What items in the XL2 setup menus would affect (invalidate or change) the WB?

I instinctively know I should setup (Gain, Gamma, Skin Detail, etc.), then perform white balance. But what if after I've set all that up, and then performed the white balance, I want to change something. Which items in the setup will (should) require that I perform a new WB?

Changing the color gain at the CCD level (R Gain, B Gain, G Gain controls in the Custom Presets menu) will definiely affect the white balance -- because that's what they're intended to do.