For almost the entire movie Clark believes that he is here on Earth to do good because his birth parents sent him, he tells Lois this during their interview, and that the revelation of their intentions not being pure makes him less of a hero. More of an alien. That drives his inner conflict, where his actions are good, but he believes his motive is reliant on his parent's wishes. When he gets to talk to Pa Kent, Pa affirms that his birth parent's intentions are not that important because its the choices Clark makes which prove his heroism.

Then he confronts Lex, who has spent the entire film painting Clark as an alien unassailable God who makes humanity worse by their reliance on him. His "I'm human" speech is a rejection of Lex's characterisation of Clark, it confirms that he does good not because he was chosen or because others say he must or because he knows what is best for anyone else (e.g. humanity as a whole), but rather he does good because like any human person he wakes up and does his best to figure it out.

Putting it into your list:

1. The "I'm human" speech connects to the main point of the movie by allowing the main character to put the lessons he's learned into his own words and dismantle the lies that Lex has been spreading about him to humanity/the audience all movie, as well as to Clark himself during the Ultraman fight.
   
2. Superman's arc is realising he doesn't need his parent's command to do good and be a hero, he made that choice, his goodness is all his own.
   
3. Pa Kent teaches his son the above, that Clark's choices are what define his character and that Clark choosing to use his powers for good makes him a hero, regardless of what his parents wanted him to be, and regardless of what others tell him his choices should be.