Niki dump:
Sensei and Niki are 31, and 29, respectively. It's been 8 years since they broke up/Sensei ghosted her, and they were dating for 5 years before that. In other words, they started dating when Sensei was 18, and Niki was 16, then when Sensei was 23 and Niki was 21, they stopped dating (which was because of the tragedy).
Niki also mentioned she wasted basically her entire 20s on Sensei, so it's likely it wasn't until the last couple years that she got scouted and became an Idol. Considering almost 3 years ago was when Sensei "disappeared" on Noriko, it is likely that it was around this time when Niki became an Idol in an effort to get Sensei to see her, no matter where he disappeared to.
Also it's been implied that Sensei disappeared because of Maya's jealousy of Noriko. Not because Noriko actually did something. I theorize it's because Maya thought Sensei would cheat on her with Noriko, and needed him to prove his love by blatantly choosing her over the Nakayama's.