Hanji, I agree with your evaluation. Ever since the start of the Airline Trilogy, PASSPO took effort to do more songs that are not focussed on the idolish side. They also incorporated the Ground Crew in their concept on regular basis. The discography is wide ranged, so no problems there. But I see no problem in exchanging her vocal lines in the songs, like Nachu or Aipon did with Mako's. Songs like Perfect Sky or Himawari will just be sung by other members. We shouldn't make the mistake of overemphasising her singing skills. Still she was one of the better vocalist in the group and it would have been untypical for PASSPO's producer to put "profit" over quality by not using her. It is one of the reasons, Aipon and Morishi have the most (and vocally demanding) lines for being the most talented, similar concept to other quality-focussed Platinum groups, like Predia.
I am just not sure why you are of the opinion that Not in Theory wouldn't have been used otherwise? But true, it is unreasonable to assume the members only writing songs because management demands it.
(I think that profit is the wrong word. Of course they do it for the money. But they also want to be proud of their songs. The creative background are rock musicians (Ground Crew, Hawaiian6). Also the members in interviews often stated that fact.)
Regarding the voices. Aipon and Morishi have good voices, though nothing exceptional either. Nachu and Sako have comfortable voices but not a very wide range. Annya recently got many songs, but to be honest, these are not that demanding either. It is more like speaking then singing (sorry, don't know how I can describe it really), though it sounds good as no other member has that kind of voice. The rest of the bunch has no quality I can see yet, even Yukki. At best they fit the usual idol category. So I am content with the way it is. I rather enjoy a good song than hearing members struggling with their singing (Rioton anyone?)