A) is the volume of air being detected in the expansion tank perhaps
displacement being caused by the rubber hoses expanding a little?

B) is there not an inherent bubble removal system designed into our system
already? Of course, the system has to be properly bled when flushing, but I
was under the impression that small bubbles would be purged out of the
expansion tank into the overflow tank, and as long as there was fluid in the
overflow tank, only fluid could return. Am I wrong about this?

C) My own thought about the chemistry of coolant: The fluid expands when hot,
hence the need for the expansion tank. Therefore it's specific gravity must
change due to temperature. Do you test the solution with your hydrometer when
hot or cold?