Forgive my poor knowledge of the game and of speedrunning in general, as I'm quite new to the community.

Is there any particular reason why the rules state that Figure-8 Circuit must be the first track selected? There doesn't seem to be any inherent reason why the tracks ought to be played in order, as long as all 32 are completed.

Obviously, starting with Sky Garden and ending with Banshee Boardwalk would be a very slight optimization, because sandbagging for mushrooms is slightly faster starting in 8th place than 1st place, plus you start in 1st place on Figure-8 Circuit instead of 8th. As it's also the most inconsistent track, resets would also be faster.

No. Everyone starts on an even playing field, the tracks will remain in that order. 32 tracks is supposed to be a large cup containing all the tracks. You don't start special cup and get rainbow road right away do ya