alright i can answer most of those
1-i arrived at 10:45 am and there was already a line of about maybe 40 people [going by sight]. but the line started forming when the hall opened which is when juniors registered so i doubt most in that line were seniors. so pretty much when it opens [but keep in mind i said i arrived at 10 45]
2-pretty much what u guessed, after you register you move up the line and you are paired with the person behind you or infront of you, it wasnt really random untill you got passed the first stage the you have to wait for another winner to be annouced and the pairing goes on from there to the next station in stage 2. then whomever wins goes up to the line and waits for another winner or gets paired with the winner who was waiting.
3-you had to beat 3 opponents to get into the finalists lounge. idk what happens after that because i left, i lost to my 3rd opponent on the closest match ever, like 1 pokemon each both under 20% hp. it came down to speed and he went first.
4-idk, some1 accidently hit run and his opponent won just by that lol so sad.
5- i heard the anouncer say something about 400+ trainers in the house, but he didnt say competing, some1 who had better hearing correct me. but if u are trying to figure out how many people will show expect ALOT, there were soooooooo many people, not like hiting the 700+ limit but still alot.