Hi,
A1: As ADS5296 datasheet describes, that filter (Figure 79) is to absorb glitches. It should be fine.
A2: Regarding how to calculate input/output impedance, please refer to attached. It shows a traditional way to measure impedance. At the same time, I'll check if we have characterization data for this with design/test team.
A3: VCA8500 datasheet specifies the output impedance 50Kohm in page 4. I'll update you if I find app note for this.
(Please visit the site to view this file)
Thanks,
KW