I know she wasn't in the RIVER perf. But the reasons are still rumors. Aren't they? Plus, the single was recorded long ago, much before the alleged Erena issue. Would the dare to freeze the activities of one of the popular members? I don't think that would be smart from the management, really.
And even so, I can't still believe Kasai or Miichan would be out. Because... They have never been out of senbas. ^^;