Answer 1

What a Super Map.

Answer 2

This map is doing me a confuse. Guessing the hitbox for entities and dustblocks aren't applied exactly at the same time? Where's msg when you need him.

Answer 3

basically supers have 2 different hitboxes; one for enemies, and one for dust and dustblocks.  whenever you super, there's a startup animation, and then the game checks for any enemies in a square radius around you; if they are in this square at this time, then they will be cleared by the super.  after this check occurs, it is irrelevant whether an enemy exits or enters the super radius; all that matters is what is in the square when this check occurs.  i can try and expand this more if it doesnt make sense.

as for dust and dustblocks, after all enemies are cleared (or not cleared), a last check is done for dust and dustblocks within the area of your player, using a circular hitbox (as opposed to a square hitbox for enemies; i suspect this was a programming oversight).  you need line of sight for dust but not for dustblocks, but otherwise this is mainly self explanatory since it's just one singular check.

this isn't so much extending a super as it is displacing the enemy super from the dust super.  being able to subtlely move down while supering is a fairly well known old piece of tech (i personally figured it out myself close to 3 years ago while working on clocktower SS) that was never really useful but this application is interesting but limited.  basically this will only be useful in situations where you need to move down and hit dust or dustblocks (NOT entities, due to how the check works) that are slightly out of range of what you super.  your downwards movement is extremely limited however, as you cannot start moving downwards until the super actually begins to hit enemies, and you will move down only around 4 pixels for every enemy hit due to hitstop.

tldr you're basically correct, supers are weird.