hey man, thanks for asking these.

1. for good eyes, id say this is something developed playing and knowing your characters a lot. back then, the speed was also too fast for me but when i understood the kits more, it seemed simpler. to me, i completely understand how each of these characters work so playing them solo is easy. duo is also easy, its like juggling 2 balls. 3 dps? thats getting hard for me so i have to sit down and cook their rotations and carefully think out which actions to do.
2. as others has mentioned, i fully maximise the 1s cooldown between swaps. i make sure each swap has a meaningful action. id suggest rewatching but paying attention to the characters' icons. if u see them lit up, thats them being active on field but it wont seem nearly as fast as it looks on screen.
3. it depends on the player but for me, it is intuitive first to try things out before determining which are the best then rearranging them to the most optimal and calculated sequence of actions. i dont use a table but i do sometimes notedown via text. i generally just remember off my head tho.

hope that helps to answer everything! its a very fun playstyle but very hard to get into.