Well, you're lucky then. Or you know the secret hand signal.
I've been locked out with plenty of time left to reach the train. (We're talking about ticket checking, not boarding.)
Again, enforcement no doubt varies by station.
Yes, as in putting your (blue) ticket into the entry gate or showing your (red) ticket to the lady who stands next to the entry gate. My experience is mostly limited to Shanghai departures (the subways are predictable/reliable so I am comfortable optimizing in order to minimize time wasted in the train stations).