On the first question, I agree with you. What you describe can be understood as the complete weightlessness of that person (equivalence principle). Imagine that astronauts can stretch freely under the condition of complete weightlessness, and of course they can also exert some force on objects.

I think there is something wrong with your statement. you said

However, if the upward acceleration of the bird is equal to the acceleration of gravity (air resistance is ignored here), it will take the bird 10 second to reduce the descending speed to zero.

I think this is wrong. When the upward acceleration of a bird is equal to the acceleration of gravity, its motion state will not change. It will still move downward at a certain speed.

If you want the speed to be zero, you have to generate an acceleration of 2 g.