Understanding a differential ADC
1 - I hope you're aware that using REFT and REFB to set your output common mode is not suggested by the data sheet. Those pins are brought out to allow filter capacitance to be added. I'd suggest that you derive your output common mode level from VREF - that sort of thing is why it's brought out. I suspect that you're loading down the internal levels, and that's why your full-scale input is only 300 mV.

2 - Are you aware that you've set up the AD8132 for a gain of -1? Just checking. This is why decreasing your input voltage causes an increasing digital output.