The usual way to provide free play on an electro-mechanical is to 'permanently' close a pair of contacts on the credit unit, which open to break the start circuit when the unit reaches its Zero position. They could either be (mal)adjusted so as not to open, or have a link placed between the wiring lugs at the rear of the switch assembly. I've heard of some owners who run extra wiring from the credit unit to a more accessible additional switch, so that free play can be enabled or disabled easily.
Exceptions to this are dedicated Add-A-Ball games, which don't have a credit unit at all. But Royal Guard isn't one of these.
If there's a working coin mechanism in place, though, there's nothing stopping you using it with a free play alteration still connected. If the chute adjustment is 1 Coin 1 Play, the coin switch should begin the start sequence.
Micro switch? Gottlieb didn't use micro switches for coin inputs on their electro-mechanicals, and probably not their early solid-state ones either.
