Topic has said it 
? Does anyone know ?
The SAT is offered once a month (except for July-Sept when it is not given at all).
You may take the SAT w/o the essay, but you cannot take the essay without the SAT.
No, if you plan to include the essay, it must be taken as part of a full test. The universities have not decided/announced how they will be looking at scores, and whether “score choice” still be a possibility. However, I imagine each school will have its own policy. The best plan would be to plan to take the entire test (including essay) each time. The essay score has no bearing on your Reading/Writing score, so don’t worry about it lowering your out of 800 score.