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PLUS Audition was indeed marketed as collaboration between BigHit x Source Music with Bang PD overseeing production, while Min Heejin would be in charge of creative direction and branding of the girl group and Soure Music taking care of management. Just like GLAM, but key role belonging to Min Heejin due to her rich experience.
There is no way creative director would leave even half of her work with others. Especially, if that person is Min Heejin, who has deep attachment to her own work and is too workaholic to let others take over her job. Also, If Source Music is using her ideas or anything created by her, they would be crediting her as it is her intellectual property, but Min Heejin is fully focused on ADOR and there is no indication that she has any involvement with Source Music GG. SouMu might be using anything but Min Heejin's work for their new girl group.
Also, you seem misinformed about Min Heejin's role in HYBE. She was hired solely for one purpose: to rebrand BigHit Entertainment into HYBE Corporation and held the position of CBO. That's what she said in You Quiz episode. But it seems like along the road, she and Bang PD exchanged ideas and negotiated to make her own agency, but first, she had to direct and debut Plus Audition GG.
After HYBE rebranding project was over, she mentioned in an interview that she had 3 missions in HYBE: 1) rebranding 2) Plus Audition GG 3) her own agency. Other than this, Min Heejin was never involved with any of HYBE labels, their branding or groups. HYBE website mentions her as "former CBO". Currently, her only position is the CEO of ADOR.
It feels a little odd to think that Source would have their name attached and be involved with the PLUS auditions if the original plan was to lose all attachments in the future once the group gets transferred to ADOR. Seems like there wouldn't have been a reason to call the PLUS auditions a collaboration project in the first place if the whole plan was to just to leave Source empty-handed in the end.
It is indeed odd that you think original plan might have been Source Music just attaching their name while expecting to lose everything in the future. Honestly, your logic doesn't even make sense. Source Music doesn't even have strong brand power to bring hype to GG, so why would they attach their name just for the sake of it. Original plan was to launch PLUS GG under Source Music, which later changed.
Source Music already had debt accumulated, PLUS GG finances were most likely taken care of BigHit Ent. In a way, It was a project to help Source Music, which had debts and was long overdue to debut group to at least strenghten their position. COVID didn't help either. Them giving up on PLUS project was probably accumulation of several factors: they couldn't handle PLUS project anymore, needed immediate results upon debut, but risk was too high + Min Heejin might have wanted full control over it. Anyways, nobody knows what went down, but seems like Min Heejin offered as an alternative to recruit Sakura, which is safer option for SouMu.
We don't know when they tried to recruit Sakura or when she agreed, it most likely happened a bit before IZONE disbandment (actually, that's what IZONE k-fans say too lol) and It was reported million times that her Japanese agency was the problem. They started forming group around Sakura and everything that happened w/ Chaewon or Minju was development of this plan. Crimson Heart twitter account was registered in July, so it was at least in works.
All of the webtoons have been only for the Big Hit affiliated groups (none of the Pledis groups got one). But Min Heejin seems like the type that she'll want the first ADOR GG to be known ONLY as an ADOR GG and not be some sort of collab group with Big Hit. The group seems to be her precious little baby so she seems to be set on having 100% control over it.
"Super Casting" project means idol groups are casted in NAVER Webtoons with Original Story made by HYBE. Nobody ever said that it was only for BigHit affiliated groups, so I don't know where this assumption came from. BigHit Music is currently 100% independent label, they aren't BigHit Ent. anymore, that's HYBE. ENHYPEN, ILAND GG, HYBE Japan BG - they all are HYBE projects and because HYBE is involved in production, they have or will have original stories/webtoons as it is Bang PD's ambition to create Marvel like universe.
Pledis groups didn't get one because they don't have strongly outlined universes, but it doesn't prevent them from getting one in the future. BTS' Webtoon has nothing to do with BTS Universe, it is completely different story. Just like BTS was casted into Webtoon, SVT, NUEST, FROMIS, etc could be too. In the end, It is up to their management if they want to. On the other hand, Source Music had storyline for GFRIEND, they will most likely continue tradition with new girl group and get storyline/webtoon.
Overall, I don't get the hype around Crimson Heart. I am not really interested in it, actually hope that you are right and it doesn't end up for ADOR GG. But even if they end up having OS/Webtoon (Crimson Heart, Red Marble or anything they might do in the future), it doesn't mean they are collab group, they will just be part of HYBE Universe. We need to leave BigHit alone, they are busy with BTS, TXT and upcoming Trainee A and other agencies' ggs are their last priority. None of upcoming girl groups will be collab with BigHit. Using same in-house producers, HYBE staff or resources doesn't make it "collab".
Anyways, I am sure both SouMu and ADOR GGs will debut before I-LAND airs in late June, we will see how it develops.