Adding capacitor for faster ADC
You should probably consider this in terms of charge transfer rather than as a transfer function.

When the Sample Switch is closed and Rs/R2 and Csample/C2 are in the circuit, there will be a current through R2 provided by C1 and R1. Because R1 >> R2, most of the current will be provided out of C1.

You need to size C1 so that it will only be drawn down to a voltage you can accept, given its discharge into 120pF. Now, 120pF is really freaking small. I'm not going to sit down an analyze it, but it should barely put a dent in 0.1uF!

You just need to take care and not call for an ADC conversion at a too-high rate, because you need to give C1 a chance to restore the charge it lost during the last conversion.

The trade-off is how long it takes C1 to charge through R1 (which determines the low-pass filter characteristic) vs. how much the voltage sags when you try to take a sample. You should figure out what's most important to you and work from there.

*the >> symbol means 'a lot bigger than'