Most of the questions you are asking you will find answers too on the clik itself. It tells you how long you need to not handle the sheep following application and how long after shearing you need to reapply, not straight away.
As far as when to spray them, it probably depends on where you are in the country. I'm in the south and have sprayed on the 1st of May because that's the earliest I've had a problem.
I'm going to use clikzin and clik this year due to the withdrawal and shearing so that I can give maximum cover, for both ewes and lambs. I've used it before and never had a problem, once applied, with blow fly. Good Luck