Can you fully seat a mag loaded with 18 if the slide is closed?

If you can, and you start with a gun loaded 18+1, will it fire and then load the top round from the mag - every time?

Manufacturers design and specify their magazines to a particular capacity for engineering reasons (apart from the 10-round nonsense sometimes, which some states are using so successfully to eliminate all gun violence within their borders ). One reason for abiding by listed capacities, which has shown up in threads on "should I unload my magazines between range sessions," is that properly designed and manufactured springs do not wear out by being compressed (they wear out by being cycled, so leave mags loaded and save a decompress/compress cycle) UNLESS the springs are moved beyond their material's elastic limit.

If squeezing an extra round into a magazine crushes the spring beyond its elastic limit it may do damage that either shortens its useful life or adversely affects its reliability. Or both.

So there usually are reasons for NOT loading a magazine beyond its stated capacity.