I am not even hung up on the suicide thing, but that's the third red herring of the same nature that we're seeing, and this makes me believe that the whole murder mystery thing is more of a diversion rather than the primary plot of the manga.You beat me to the punch. I just started to suspect something similar.
Not that she killed herself, but everything is sort of an accident, maybe someone was involved but no one knows or was intentionally left out by some reason, and these two will just go on and solve the problems for all the classmates - and along the way, solve the complex current Miyu has >> That's my whole series plot prediction at this point.
Also the manga is kind of a light black comedy, it has this sardonic tone, confronting an actual murder mystery would ramp up the drama factor and detract from its comedic value, so I expect the resoultion to be something anticlimatic, like suicide or accident or even Yuushin being the culprit.