I like this theory. It would explain why Kabura got her on the cruise even though Souko could virtually do nothing (She has to make use of what she has even though its broken). Additionally, Kabura may have also intentionally omitted this fact to the other Valedictorians, as Kurokiri made a comment about how one member *didn't care enough* to appear. They may not be aware, and I think Kabura did so because they wouldn't be happy with a team member who's mentally incapacitated.
However, it's hard to believe that Souko would graduate as a Valedictorian given that she was frantically gambling AND losing during her later time at the school. That and she got hospitalized at the end of it. Im actually not too sure about the timeline of events but I think the flashback chapters shown is her first time in Hyakkao, meaning she lost Yumeko in the same school year, which lead to her hospitalization (not even sure she finished a year in that school). There really was no mention of her graduating.
Nevertheless, she's shown to be an excellent gambler and I wouldn't be surprised if she got elected to be a Valedictorian. It would be really cool I think (even though shes basically catatonic now)
Kinda thinking the 7th member is Souko.
I don't know if i have missed it but I don't think it was shown how her life at school ended. Only flashbacks of how she continued gambling. Maybe she was a valedictorian.