Yes, that's why you have to be careful.
It prevents whatever it's blended in from freezing when it gets cold. For cars, it also doubles a little as rust inhibitor.
Water expands when it frieze's. Well your engine has areas in it for coolant to travel and cool your engine. If the coolant frieze's and has no room to expand in those designated areas it can crack...
Yes, anti-freeze can kill you if you drink it.
Anytime as long as you are adding it to the reservoir. Never remove the radiator cap when the engine is hot.