I can only answer number 2. I don’t think you pick a specific pace. Just divide the 200 time by 4 you’re if doing 50s. I think that’s what you were asking.
Why are you doing them off the blocks? It seems like it would be hard to keep the rest to 20 seconds if they are climbing out and doing a start each time.
"Most people have the will to win. Few have the will to prepare to win."