True. I'll agree these are all good points and, in retrospect, the technology jump is within a reasonable time frame. I still feel awkward accepting that the world can dismiss the actions of the Fire Nation, which include the genocide of an entire race, and can be forgiven enough to allow for the cooperation needed to build Republic City. Even if the blame was transferred solely to the previous Fire Lords, the people of the Fire Nation were still following them loyally.
But, they also did have a lot of resources and technology to offer so maybe it's not all that crazy that they could overlook what happened and for forgiveness to come about so quickly.