You can always use a higher rated switch to control a smaller load. You can not use a a lower rated switch to control a larger load.
It can be done although it is bad practice provided the load doesn't continuously require more that 15 A. You should then use a 15 A fuse to protect the switch.
Yes- The switch just has the ability to carry a higher current then the normal 15 amp type.
no harm done switch is rated for up to 20 amps
No, it shouldn't be. Purchase a 20 amp rated switch.
Yes, but only one per 20 amp outlet.
How to Change a 13 Amp Fuse
Adding More HDMI Ports to Your Home Theater System
Replacing Neff Cooker Hood Filter and Light
Elizabeth Benedict Talks About Mentors, Muses and Monsters