After a show opens is usually when Tony voters are invited to see the show. There is no way that they can wait until the last week of May to see them all...because many of them will have closed by then! Just this season alone, BIG FISH, FIRST DATE, BETRAYAL, SOUL DOCTOR, BRONX BOMBERS, TWELFE NIGHT/RICHARD III, etc. have already closed...and that's an abbreviated list from the top of my head.
As to your second question, there is no way to predict the likliness of when a lead will be out of their show. The exception is when a lead actor has an alternate who goes on at specific performances during the week, as is the case with A NIGHT WITH JANIS JOPLIN.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle
