Welcome to the forum!
I doubt that 20 volts will harm the switch motors, as they are receiving power for only a fraction of a second when energized. When these switches are used in a command control environment, they are subject to 18 volts of power from the command system setup. The only components that would be stressed would be the bulbs in the switch machines and the controllers. I would recommend replacing them with long life, cool operating LED's. These are drop-in replacements for the bulbs, either screw-in or bayonet type.
http://www.autolumination.com/trainbulbs_leds.htm

If you are running only 1 train with the KW transformer, you could use the 'B' throttle to operate your switches, and adjust it to a lower voltage in the 14-18 volt range.

Another option to think about would be to get another transformer just for your switches, like a 1033. Properly phased to the KW, you could again dial in any voltage to operate the switches smoothly and consistently.

Larry