And I think it's a bit deluded to say that other Eggs in Ongaku Gatas are somehow much better than her vocally.
OG has the appeal, but not the vocals. There are people in OG who are vocally stronger than Mano, but that's not saying much.
Back in her graduation, Konno didn't have the beautiful vocals she has know (see her Uta Doki appearances). So there's still chance for Mano, who will probably start getting major trainement and attention so she can have the best case scenario before she debuts, and maybe a few years later she can hold a concert by her own.
How much has Koharu's lack of talent in singing held back her career? Well?
Kirarin has the whole anime thing going for her ("solo" singles, merchandise, all the extra-MM stuff). Koharu is popular as Kirarin. Not so much as Koharu (specially outside Japan). Do we even know what kind of image will Mano have? Aimed at kids (which doesn't insta equal anime stuff, she may get kiddy songs who aren't anime songs), sugary sweet loli stuff, maybe a continuation of the upbeat OG songs? In jpop, an artist's image IS important. In Mano's case... well, wota like young girls. That's why her merchandise sells good. Though if UFA wants to make her a long-term soloist, they better not cater her exclusively to the wota.