For a large part of the series, Nanami is annoyed that Tomoe is seemingly a shameless flirt. What she doesnt realize is that Tomoe only "flirts" with female kami, chiefly ones that are extremely powerful. Besides the fact that theyd be a huge pain to fight, the other kamis already look down on Tomoe for being an akayashiif he gets involved in a battle, especially with a female kami, hell inevitably be blamed and it will just be held against him. What Nanami is perceiving as flirtiness is actually Tomoe being prudent and trying to avoid getting into hot water with the other kamis.
In chapter 84, Akayashi!Tomoe asks a raccoon prostitue if she would sacrifice herself for him in order to test her loyalty. 500 years later, Akura-oh (in Kirihitos body) orders Yatori to cut off his arm for the exact same reason. It goes to show that these two are more alike than they may realize.
Digging deeper, the incident with the prostitute occurred when Tomoe was beginning his arc toward becoming a genuinely caring person. The arc was technically kicked off by Nanamis interference, and her initial defiance of him is what made him consider the concepts of love and loyalty in a serious manner for the first time. When Akura-ohs incident occurs, Nanami has also interfered with his life in a way that makes him consider those concepts with more weight than he had beforehand, signifying that he was starting down a similar character arc.
Its fitting that Yatori would ultimately take over Sukes body, since theyre both the weak links in their respective worlds; additionally, theyre also the ones that are emotionally disturbed enough to eventually turn to murder (or, in Sukes case, attempted murder) in their desperate search for happiness.
Nanami always assumed that she would never get married, because the curse on her family meant that 1) her marriage would be unhappy, 2) she would only be able to bear one daughter, and 3) that daughter would grow up to be in an equally unhappy marriage, thus continuing the cycle. So even though Nanamis marriage to Tomoe was perfectly happy, she probably had nagging doubts that the curse was going to take it all away from her at any moment. Which explains why she was so elated when she discovered that she was pregnant with a boyit proved once and for all that the curse was broken!
Its great that Nanami and Tomoe eventually return to the shrine permanently, but imagine Mizuki and Mikage having to watch their close friends secede to old age and then die right in front of their eyes. Mikage is certainly strong enough to handle it, but Mizuki has always been emotionally fragileto see his dear Nanami-chan grow old and die could destroy him.
Kotaro and Himemikos fates are more or less left to the readers imagination. However, remember that Kotaro is still human, and Himemiko is the kami of the Himemiko river. As kami of the river, Himemiko cant abandon her duties by becoming human, because that would cause the whole palace to collapse and doom the river itself. Since Himemiko cant become human and Kotaro obviously cant become a kami, then Kotaro will eventually grow old and die, and Himemiko will be alone without him. Its outright stated by Mikage that kamis who lose their human lovers to death eventually wither up, unable to move on from their lost love. Imagine what will happen to Himemiko