In the anime, if the SPACE owner was so against Kasumi's actions of going onto stage and singing "Twinkle Twinkle Little Star", why in the world did she let it go on for five minutes straight, instead of dragging them off stage?
Because 1: The audience enjoyed it, and 2: Glitter*Green were late, so someone had to buy time and stall the audience until they got there. Forcing Kasumi off the stage would have made the owner look bad in the eyes of the audience, and at any concert, the longer the silence between two bands is, the bigger the chance that people leave, even if they actually paid. People leaving means the venue makes less money on drinks (and in this case also food), and possibly giving the venue a bad reputation. There was no way it would have been in the owner’s interest to force Kasumi to leave the stage. In fact, Kasumi did her a huge favor without realizing it, even if she didn’t mean to.
How did Saaya manage to pretty flawlessly perform "STAR BEAT!" in episode 8, after literally zero practice, having only listened to the song once through earphones, and after not having drummed for years? I'm not sure how much drumming is akin to riding a bike when it comes to never forgetting how to do it. But trying to perform along with others to a song that you've not practiced and are completely unfamiliar with, while trying to readjust yourself to the basic ropes of drumming again at the same time, seems pretty much impossible to me.
The most unrealistic part would be the fact that she hasn't played the drums in a long time. It'd definitely be possible to reacquaint yourself with drumming swiftly, but trying to do that while simultaneously attempting to keep up with a performance, and pulling it off perfectly, would be near impossible. Her being able to drum to a song she's unfamiliar with would be possible, although it'd be difficult after only hearing it once, and being told nothing else about it, while also reacquiring yourself with your innate talents. This could've been intended to demonstrate just how good Saaya is at drumming though, since it's not like it's flat out impossible.
For one thing, it is implied, if not stated directly, that Saaya left her previous band no more than a year before joining Poppin’ Party. The flashbacks also indicate that it can’t have been a long time, since her appearance hasn’t changed much since the flashbacks. For a musician, who hasn’t played their instrument in a year, their skills would start to deteriorate, but not to the point where playing would be completely impossible. So even after a year, Saaya would still be able to play just fine. Maybe not as complex rhythms as she actually does play, but at least rudimentary stuff would be no big deal for her. And having been good once, it will be naturally easier for her to regain the skills she once had, rather than actually having to relearn everything from scratch.
Just how skilled are Glitter*Green? The only time they actually perform on-screen is while playing “Kirakira Boshi ~Hajimari no STAGE ver.~”, where they manage to, on the spot, come up with a pretty damn good rock-arrangement of “Kirakira Boshi” (“Twinkle Twinkle Little Star”), with no sign that they have actually talked it over or anything. It may be an easy song that anyone could play in a normal arrangement, but the “Hajimari no STAGE ver.” is a bit more complex, and actually has each instrument play unique lines, so the instruments don’t clash and drown each other. Roselia may be the most technically advanced band in the franchise, and Afterglow may have the most energetic performances (being a punk band, they would naturally play with more energy than Roselia), but Glitter*Green might just be the one band with the strongest grasp of music theory, since they are able to completely re-arrange a classical song to a modern pop-rock arrangement in no time. And make it sound easy.
After the 1st Anniversary Show blunder, why did neither Arisa, Rimi or Saaya consider suggesting that Tae brought Rokka to the next Raise A Suilen rehearsal, so Rokka could audition for the band as a full-time member instead of Tae? They all saw just how good Rokka is, they know she’s looking for a band and wants to help Popipa, and especially when they learn how aggressive CHU2 can be, they should at least have considered the possibility. It could have saved them a lot of headaches and could actually have gotten them on better terms with RAS than they ended up with...
As you say, CHU 2 was highly aggressive in her pursuit of Tae. She asked for Tae specifically. Saying, "Don't take her, take Rokka instead" could have been seen as an insult to both Tae AND Rokka; Tae because it would seem like they were selling her skills short and Rokka because it would seem like they'd rather ditch her than lose their friend Tae. Given how CHU 2 approached them, they weren't in a frame of mind to offer substitutes, anyway. She came in aggressively to snatch a valuable member away from the group precisely because she wanted to eliminate the competition, not because she wanted to offer a promising guitarist a chance at a good career, no matter how she might have phrased it to Poppin' Party.
Where on earth does Poppin’Party get the money for their equipment? We know that Arisa is just from a wealthy family, and Tae worked at Space to save money for her guitar (according to the game, though the anime indicates her mother gave it to her). That’s fine. But what about Rimi, Saaya and Kasumi? Saaya probably doesn’t make a lot of money helping out at her family’s bakery. And Kasumi couldn’t even afford to pay the withdrawal-fee on the guitar she bought from Arisa. So where do they get the money for their amplifiers, pedals and spare strings and such? Do they just use whatever amps people happen to hand in to the pawnshop, until a buyer comes along?
Why don’t Ako and Tomoe share the same drum set? It seems like a waste of space and money considering they’re siblings.
Simple, really, Roselia's songs require a different setup for songs then Afterglow's do, and since Yukina wants Roselia to be really good, Ako having the setup she needs for home, will help her get better.
If Kokoro's family is so rich, why does she go to public school?
It's probably because Kokoro wants to be at Hanasakigawa, perhaps finding it a lot more fun and exciting than an exclusive private school would be.
