So if orbiting astronauts are in constant freefall around the earth...?

Whenever the shuttle achieves orbit, its velocity remains constant (except for minor navigational corrections with the thrusters).

They 'bonk' around inside the shuttle as they push themselves from one place to the next.

To be strict, they are not in constant freefall. They are in a 'microgravity' environment. There is some gravitational effect and they are not completely motionless objects - breathing and moving their limbs will have consequences on what's happening as well.

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As for your car analogy, that's Newton's first law of motion. A body remains at rest or in uniform motion unless acted upon by another force.

Initially as you accelerate towards 60mph, you will feel the effects of the acceleration until the car reaches it's 60mph velocity and your body is in sync with the movement of the vehicle. If the brakes are forcefully applied and you are not restrained, then you will experience deceleration as you are continuing at 60mph while the car is maybe doing 55mph or 50mph. When the deceleration of your body reached 50mph, it will feel fine again as it is in sync with the movement of the car.

Yes Yes

(okay a few more words) Yes, since they and the ship and all the stuff inside and tossed out the port are falling together around the world, only when the thrusters or engines are used does the ship change course and anything not fastened down gets bumped by the walls.

That's exactly right.

In fact the way I think about it is not that gravity is itself a force, but rather gravity defines the acceleration frame relative to the centre of the universe that requires no force to maintain it.