To have it work that Chie sabotaged Mirror/Ƨoul prior to them reforming with Minori, Chie is a 2nd year (who looks young...if Nico can look like a baby then so can she), Mana and Yasu are 3rd years and Minori is a 1st year. Ayako and Emi are also 2nd years, Kyo is a 3rd year, Tadashi is also a 3rd year.
As for the Boyband Idols, well you got me. Young Innocent is definitely a 1st year but I keep pondering for the others between 2nd and 3rd and being unable to decide (so far I'm leaning towards them just all being 2nd, which Strong Silent technically being a 3rd year but she had to repeat a year because of her injury. Not sure if that stays though, like I said, I keep swapping them depending on how I feel on any given day.)
Mana was technically the one who asked out Tadashi since he would never. It was quite informal and spontaneous though.
You might think I'm done expanding the "lore" if you wanna call it, but I have not yet mentioned that almost since the time of her creation, Kyo has an older brother who is similarly chill and multitalented and who helps Ayako with making her costumes.
Young Innocent really just wants to get along with people and is naive and easily deceived. Naturally she's the easiest prey for Minori and Chie, who also have a mental battle of their own.
Chie routinely tells others idols "Good luck" before a show, an intentional subtle snide remark since there is the superstition that wishing somebody Good Luck before a stage-performance means bad luck.
Pretty Girl and Goofball have a falling out at some point which threatens the group. It becomes a bit of a thing for a while. I'd like to draw some scenes from that but it'd be so much work so maybe I'll just ramble my thoughts on it instead at some point, depending on if people are curious enough (even if not, I might do it anyway, try to stop me)
Oof I'm running out of trivia at the moment so here's a doodleroo of Young Innocent getting a paper ball thrown at her