Q1. Your understanding is correct, you can also disable 'EMC noise immunity' if it is not required
Q2. Yes, the refresh time will the total scanning and processing time of sensors or 20 ms (typical) whichever is higher. You can refer the MBR3design guide for the complete layout guidelines in the following link
Q3. This can't be reduced below 20 ms. It is configurable from 20ms to 500ms
MBR3 will be more optimal for configurable low power button design, if you need customize feature you may look PSoC 4 based solution.