If the piston is left above the exhaust port, then some fuel/air mixture in the cylinder can fire when you put on the glow plug battery connector.

The inserts are used to 'tune' the engine for different types of running. Without them, the engine won't draw fuel well or respond to throttle.

The book values for needle setting are only starting points. You have to work with them to get them set to the way you want the engine to run. Where they wind up is always correct, as long as the engine's running well.