Rei was far from their main composer, actually... Eiki and Kou are the main composers in general, Eiki having studied music in university and Kou having done it as a hobby for years before he even started his first band. I believe he plays some guitar/bass too. To be honest I have a feeling Kou is the driving force coming up with guidelines, concepts and basic structures while the other members always kinda trickled along, Eiki being the biggest help mainly doing most of the composing/technical bits. I mean Kou is even in charge of Art Direction, he just takes on everything all the time, haha. I believe the one song Nao was completely in charge of during the Shounenki era was ムービスター (WEAKNESS MY BLOOD B track) which was ... I guess while it was a fun live track it was pretty lacking over all and didn't fit the rest of the songs/theme whatsoever.
They disbanded because Rei wanted to leave the band for personal reasons, and they didn't want to continue Shounenki without Rei. Simple as that. I guess they decided to take the opportunity to change up their concept (as you wrote) since Shounenki, as amazing as they were never really seemed to get anywhere.
I was a huge Shounenki fan and went to probably around 60-70 shows before they disbanded but as much as I am trying it's been really hard for me to get into this new band due to the extreme change of concept... I used to love Kou's lyrics and blogs but it's like he's no longer there as he's taken on this character singing about the mental struggles of bangyas which unfortunately seems to be working in regards to people actually attending the shows. As much as I believe they'd like to do nation wide releases, they won't for financial reasons. People want the music? Then go to the lives, buy CD/goods/chekis and help them with their activities-kind of strategy. Whoever uploads these to YouTube, while I too want to be able to spread their music, you might be hurting them financially as Japanese fans won't bother coming to the shows since they already got the music online. The band even tweeted they were sad to see it happening. I understand why it's frustrating for you guys though, especially if you can't even access the lives. Just know I'll happily pick you up copies of their releases at any time and send them over, just message me on here!
I believe their label CRIMZON is currently throwing all their money to Purple Stone and it's pissing me off seeing them release PV after PV while Amabo is just left to struggle on their own. I went to the oneman last week and there's probably only like 20 people from the Shounenki days still there while the rest is new fans. I dunno obviously if they're making good money but... I dunno. I hope it's going well? I was happy seeing so many new fans there but the capacity of the venue they played is smaller than what Shounenki used to play. I feel it's just such a waste to have such talented musicians flop around doing whatever this is. This concept won't last. You can only make that many songs about being a girl on the side, lol. We're gonna get bored, THEY're gonna get bored.
I really hope they can turn this around somehow... I'd hate to see them just fizzle out to nothing. Especially Kou and Eiki. They're far, far too talented.