It's possible that Nick telling the story was his way of making sense of what he experienced and trying to understand the complex relationships and motivations of those around him.
Yes, Nick shows some interest in Jordan. Their relationship is complex and involves elements of social interaction and shared experiences within the story's context.
One reason could be that Nick's character provides a contrast to the other characters, allowing for a more comprehensive view of the story's events and themes. Also, his presence as a narrator helps the readers connect with the story on a deeper level.
Nick's final words are a condemnation of the upper - class society he has witnessed. He realizes that the American Dream as represented by Gatsby's pursuit was a false one. Nick says that Gatsby believed in the green light, in the orgastic future that year by year recedes before us. It was this dream that led Gatsby to his downfall, and Nick sees the tragedy in that. The wealthy, like Tom and Daisy, were just using people and then discarding them when they were no longer useful, and Nick was left to pick up the pieces and tell the story.
Gatsby finally tells Nick the truth of his life - how he came from a poor background, fell in love with Daisy, and did everything possible to become wealthy and be worthy of her. It's a tale of hope, love, and deception.
Nick has a complex perception of Gatsby. At first, he's intrigued and impressed, but as the story progresses, he sees the flaws and tragedy behind Gatsby's facade.