Bushranger

I am trying hard to understand. Consider a rocket in space that suddenly accelerates (thousands of miles per hour). The rocket cabin (a cylinder) is filled with air. Is all the air then suddenly "pushed" to the rear of the rocket due to "bodies that are at rest tend to remain at rest", and a vacuum formed at the front of the rocket because all the air is not in the rear? This all seems counter-intuitive to me.

Consider a space ship in near/far Earth orbit. Suddenly accelerates to thousands of mile an hour instantly. Would the astronauts be slammed to the rear of the vehicle or would the fact that they were only subject to micro gravity (mass but little/no gravity to act upon the mass) they would not be so effected? In short, in the absence of gravity (I understand that there is "microgravity" in Earth orbit) is law of inertia (bodies at rest remain at rest), in effect? Please explain it to me like I was a six-year old.