I see Mike hasn't had a chance yet so here goes;
Question 1. Safety lanyard= Kill switch. Or, disable the ignition by disconnecting the coils.
Question 2. Any way you can...YES, disconnect the throttle cable at the carb linkage so that you can hold the throttle completely wide open. Use some wire or vise grips or something if you don't have a helper. You will NOT get good info if you don't hold the throttles open all the way while cranking. Make sure fuel is disconnected as well.
Question 3. The fact that the engine starts and idles well is a TYPICAL indication that the engine is sound. However, if you get questionable results from a compression test, checking the timing is the first step of diagnosis.
Question 3, addendum A. A leak down test is nothing more than putting compressed air to a cylinder "parked" at top dead center compression stroke via the spark plug hole and then determining where the air is escaping. If it leaves via the carb inlet, then you have poorly seated intake valves or bad tiiming. If it exits via the exhaust, then you have poorly seated exhaust valves or bad timing. If it leaves via the crankcase, you have leakage past the piston rings.
An internal combustion engine is, first and foremost, an air compressor. Of course, some leakage is allowed but not much.
Harbor Freight tools sells an acceptable leak down gauge for under $50 but you don't necessarily need a special tool. It makes not putting so much air in the cylinder that the engine cranks easier as well as providing an easy to read leakage percentage assessment aid.
As far as your compression gauge, the only way to make sure it is accurate is to compare how and what it reads to another known accurate air gauge while applying compressed air.
Good luck.