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The 49.9Ohm resistor in the first drawing is part of the dual feedback topology, and is used for amplifier stability. You can find more information in the following TIPL topic: Stability - Isolation resistor
Regarding your second question, this filter provides a 4th order low-pass response at the modulator frequency. The response can be found using multiple filter design tools, and is then verified in SPICE simulation.
Why do we care about aliasing suppression of fMOD? The digital filter will attenuate alias frequencies from the filter cutoff frequency up to fMOD-fDATA/2. At fMOD-fDATA/2, the digital filter will have 0dB attenuation, passing these signals through. The analog input filter is then needed to attenuate these high input signal frequencies. Figure 7-17 in the datasheet shows the digital filter response from 0Hz up to fMOD.
fMOD-fDATA/2 is where the response quickly increases back up to 0dB (gain=1).
