Also, why does Zuko always seem to look angry? It's not just his eye; it's his whole facial expression in general.

Did you not see ATLA? He was pretty much always like that.

Eh, maybe it just feels more defined here, plus in ATLA he usually had legit reasons for looking angry.  I mean, in the one episode of Korra he was like "Hey nice to meet you, I am going to angrily glare in your general direction."  Tho I did appreciate his look of surprise when Korra mentioned Iroh.

I would probably be more concerned if he wasn't stern and cynical. Plus he's like what... 85 or 90? I think he's earned the right to be a brooding old man at that point.

Zuko is just grumpy because he didn't get to talk with his Uncle in the Spirit World.

Also that he can't go on a life-changing field trip with himself.

I think Mako using lightning again symbolizes how he's finally gotten his act together. Remember back in ATLA, how Zuko couldn't bend lightning due to all his emotional turmoil? Mako could bend lightning right off the bat, when he was in stoic older brother mode—but the emotional turmoil he experienced as a result of breaking up with Asami and then Korra (manifesting in his awkwardness around them all season) seems to have hampered his ability to do so. He specifically seemed to have snapped out of it when Korra hugged him before they went their separate ways, reminding him that even if they were terrible for each other romantically they were still close friends and his past mistakes didn't change that.

Except Mako's firebending ability has not been hampered at all, and he's held his own moderately well in various fights and stuff (he especially showed off some skills when he chased Kai onto the train.)  I think it's more that the writers just kinda forgot he could use lightning until the last episode.    And awkward relation stuff != turning against everything you believe in, ala Zuko.