The amount of water/
that the system takes I believe is 7 Litres (According to the manual). I initally replaced only 1 litre and after discovering the 50% reduction I topped it up with approximately a half litre. I have been monitoring the levels to ensure there is no leak and I will continue to do so. So far so good.
The other scenarios that you describe sound a little more ominious, I believe that its safe to assume should the levels remain constant I need not worry about these?