Electrical – get a switch for a 30A circuit

You are correct on both counts:

• There is no problem with using a 20A switch on a 15A circuit, as the breaker will trip and protect the switch from over-current.
  • Using a 15A switch on a 20A circuit is potentially dangerous though: the circuit may draw up to 20A, but the switch will be overloaded at that point which may cause it to overheat/melt/start a fire.
  • Really this is the same as wiring: you can use larger wires (with higher current ratings) but not smaller wires
• A 20A outlet is not allowed on a 15A circuit. It's not really "dangerous" since the breaker will (should) trip at 15A anyway, but it is bad to knowingly install it this way.

The ONLY way this would be code complaint (legal) and safe is if the switch were solidly mounted to a metal plate, which would in turn be mounted to a grounded metal box. If you have plastic boxes there will be no way to ground the plate through the box.

NO! Soldering is NOT an option as this is expressly forbidden in the code. You MUST have a solid mechanical connection even before soldering. Having said this there is no means or provision for any sort of mechanical connection to that switch or to a switch plate, unless of course you nut and bolt the ground wire to the plate, but this adds a visible screw head to the finish side of the plate.

Almost certainly you'd need to get a stainless steel blank plate and drill your own hole.