re: Solar sail
It would depend on how far the sail is from the observer and how inclined relative to the sun and the observer the sail would be.
Just like the Iridiums or ISS, the magnitude would change enormously in a few seconds, if it's in a LEO; if it's very far, it would be slow and change slowly, but the magnitude would also be different. I think we can only see it happen in order to judge its magnirude.
Raoul