You are very right with the theory. Helium gas is lighter than air. The balloons when filled with Helium or hydrogen gas, tend to deflate quickly than normal air balloons because the air pressure outside of the balloon makes the helium balloons deflate quicker. You must also have noticed that the mouth blown balloons remain as they were as long as you don't deflate them yourself, or they get a hole in them.
Hibrida: The molecules can only escape if there has been a hole punctured in the balloon.
Yes , they do due to less mass.