The Legend of Korra comics are definitely worth a read. They capture the essence of the franchise and bring fresh perspectives. The quality of the storytelling and the visuals make them a great addition to the series.
The 'lemon' aspect might be a bit hard to come by in a clean way as it often involves inappropriate content. But for normal Korra and Mako fanfictions, you could also try FanFiction.net. Some stories there might show their relationship developing, their adventures together, and how they support each other, which can be really interesting to read.
The number of hours that a good ending would take depended on one's personal taste and preferences. He could go to various online novel platforms, such as Qidian Chinese Network, Xiaoxiang Academy, etc., to search for different types of novels such as fantasy, romance, science fiction, etc., and then choose a novel that was suitable for him to read.
The main theme of the story could be described as the sudden realization of freedom and the subsequent mixed feelings. It shows how a woman's perspective on her life changes when she thinks she's free from marital constraints.
To be honest, I don't have an exact figure. But you can look up on popular comic book databases or ask in comic book enthusiast communities to get a clear idea of how many The Legend of Korra comics are out there.