Yes, if you get ram for the same price, its better to get the faster one at lower voltage (with same latencies).
Yes, its possible to underclock, but you might not be able to achieve CAS 8
Its the same, if both of the sticks can go up to 2133, then your overclock room is the same. If you get one stick at 1600 and another at 2133, and then found out that the 1600 doesnt go all the way up to 2133 then obviously your OC room is smaller
Regardless, Sandy Bridges doesn't show much (if at all) gains after 1600 Mhz, CAS9 so all your theoretical questions are more or less useless with regard to S-B