"Not necessarily optocouplers, I might use a HCT14 and dual diode protect the inputs."
How about using a pic microcontroller?, it has built in dual diode protection and can sink up to 25mA current per port pin.
The sink current does not matter, you are using an input. The protection diodes that are real on HCT and parasitic in most uCs is what matters.

I will add a sereis resistor and a small capacitor to each input, would this get red of the HCT14 and the diodes?
Not the HC(T), but I believe you can live without the diodes

I always use a ground wire to microcontroller board and never rely on chassis ground, but every 2 or 3 switches share one ground wires because I don't have enough wires in the cars loom, is this enough?
should be


Erik