Flying bird weight

Makes no difference, other that it would be very difficult for a helicopter to fly in that small an enclosure.

You can easily analyze this with centers of gravity. If you have a closed box (no effects with the outside world other than through the scale it sits on), then anything that does not actively accelerate the center of gravity (CG) makes no difference to the scale. In other words, a helicopter sitting on the bottom, or flying near the top, or moving at a steady speed registers the same to the scale.

When the CG is accelerating there must be a net force on the box, so the scale will read a change. For example when the helicopter takes off and moves to the top of the box the CG of the total box + helicopter accelerates up. This requires an upward force from the scale, so the scale will read heavier than normal while it accelerates up, go back to normal while it moves at a steady speed, go lower than normal while it decelerates to a stop near the top, and finally return to normal while it hovers near the top.

It can’t make a difference - if it were otherwise, this would imply the possibility of creating an inertia-less drive system (with which we could visit the stars).

It’s possible that with a huge box and a tiny hummingbird, the measurements we try to make are below the level of experimental noise and error, but there are no free lunches.

No change. Regardless of altitude, the helicopter is pushing air downward. That moving air won’t stop moving downward until something else pushes upward on on it; “something else” is the floor of the box.

If the helicopter is high enough, you ( a single-point observer inside the box, far below the helicopter) may not notice its downdraft. This is because the downdraft has been dispersed/slowed by the surrounding atmosphere. All this means is that the downward force is being exerted over a larger area of the floor of the box, by a larger, slower-moving downdraft - but the total force will still be 0.5 pounds.