I think you're overthinking it. I'm honestly confused about what you're trying to say.
Feel free to clear up my misunderstandings or any facts/correlations I'm remembering wrong.
Did Kaori have an illustrious solo career outside of SanHora when she started working with YK ("tsubasa" in 2005)? Unless, rather than their debut with YK/FJ, do you mean the lives & "Parallel Hearts"+ era of FictionJunction? In that case Wakana and Keiko already were doing Kalafina at the same time so uh? And when Keiko and Wakana didn't know if Kalafina would continue beyond KNK they DID try branching out. We wouldn't have gotten Azuki Hino songs without Keiko trying... but then Kalafina continued...
PS. rito had a solo career prior to FJ? Serious question.
Also I think the youtube link you posted has no comments that we can address in any discussion.
edit:
Looking at vgmdb at least, it seems "tsubasa" was before SanHora even and Kaori had performed on one song in 2002 prior to that.