Good answer, except for one thing.

The clunk should NOT be able to swing forward in a standard tank. Very often, tank MFGs will include a stiff plastic tubing to be inserted in the tanks feed line to prevent the clunk from swinging forward and getting stuck.

As far as the feed line sucking air while nose down, it just doesn't happen (at least not in most cases) due to the fact that the plane is moving forward, thus pushing the fuel to the back of the tank. If some air IS fed into the line, it will pass through the carb much faster than fuel will, so you may get 1 or 2 lean "pops" out of your engine, but when you consider how many "pops per second" you're getting, they will go by in the blink of an eye and usually won't even be noticed.