Season 2 was planned later on, probably halfway through what was believed to be the one and only season. You have to remember this sort of show takes years to produce and they were on tight budgets, deadlines and more than likely had other projects to work on. When we got the news of both seasons the creators REPEATEDLY stated that the two seasons were created independently from one another because of this sudden pick up.
As for the Amon anger, upon reflection is just a lot of us fans raging that "My idea was WAY better" as if we wrote the show. Its just a case of fandom wanting things to go their way not unlike shipping. In fact I daresay Amon's identity was an even bigger issue than shipping. We all had our theories, 2nd/3rd guesses and hopes. But that wasn't the story Mike and Bryan wrote.
Amon is the brother of Tarlokk, Sons of Yakone the Bloodbender. A character that we didn't expect. But to say he was underwhelming just because he wasn't who you wanted him to be is pretty much pointless. As a villain he was constant in the season, he was always there whether in person or a the aura of fear he radiated, far more so than Ozai. He was unfortunate to once again have to deal with a type of bending that he probably isn't familiar with at all, which was the challenge of the entire Fire Nation in ATLA. That's right, Airbending.
So the fandom's imagination took a life of its own. It happens. Way too often if you ask me. But does that diminish the show? No. There were a lot of restraints, both in terms of production and censors but I think Mike and Bryan did they're very best to deliver a story worthy of the world of Avatar. Remember, this show is still their baby and thus far they've done all they can to respect the wishes of the fans.
I'll give my rating for the entire season here
9/10
Why 9? Because it was short. Everyone's going to have opinions on the matter. Part of being a fandom, but if you are writing off the mainly because you didn't like the reveal is pretty damn petty.