Well I finally got back around to probing about my initial board, and I think I found another reason to respin this board. I think my differential amp is hooked up backwards! While probing I found that the output of my circuit was saturated right at five volts, which is the output of the differential amp that compares my references voltage divider to the thermistor divider. I'm not big on amplifier theory but if memory serves me, feeding back into the non-inverting input is probably a good explanation for the output saturation. I haven't given up yet, but I do have to rethink the design a bit. More to come.