Dummy Switches - Charge Injection and Capacitive Feedthrough
1) Because the charge injection on M2 is the opposite sign from that on M1. "Notice that the clock signal (on M2) is the complement of (...) M1".

The charge injection is basically capacitive coupling from the large amplitude voltage step on the gate, to the channel (divides between source and drain ends).

By driving M2 with the opposite polarity voltage step, you can cancel most (almost all, with care) of it out.

2) Because when M2 is off, it is still shorted out, and the short conducts.

I wish I had seen this trick earlier ... I could have used it in 1983 or so...