[DonRotolo]

12V @ 40A = yes. The main breaker, rated at 120A, can carry twice that for short time.
No, you don't necessarily need a switch rated for 40A.
Yes, a plain light switch will work for several hundred contact closures before it fails.

You also do not need more than 1 switch to reverse polarity; you need a DPDT (Double-Pole Double-Throw) switch instead.

All that being said, your very best bet is to not power a CIM with full battery voltage. Not at all harmful, but full speed isn't always the best operating point. Better is to power it through old FRC hardware, available from many teams and on eBay cheap. Look for older Power Distribution panels, Victor/Jaguar motor controllers, cRio or IFI robot controllers and so on.