The ADC problem as well as cost of the voltage level shifter leads me to think having a co-processor is the best solution. The last two posts look at 8 bit and 16/32 bit processors I could use for this purpose. For firmware update, I want the co-processor to have USB capability. This would allow me to get rid of the USB to serial bridge chip.
I ended up with Microchip as the manufacturer for both architectures. The PIC16 line of controllers and the SAM32C21 line of controllers.