JGL,
I am interested in your suggestion to eliminate the shift registers and Schmitt triggers from the input module. Have you thought about using an Arduino Mega instead of the ProMinis and the I2C bus. The Mega has a total of 70 pins and can be bought on eBay for $7. You could control 14 switches with one Mega by using 28 pins as inputs and 42 pins as outputs (28 for the switch relays and 14 for the leds).
I did a small test with an Arduino controlling 2 switches, and it seems to work fine. I just used the optocoupler circuit that you diagramed in your post above as the input and then connected the Arduino outputs directly to a relay board. The advantage of this setup would be its simplicity. The optocoupler boards can be assembled quickly and the only other connections are jumper wires going to and from the Arduino.
