Damn it, so as predicted she really did run away from a creepy, abusive stepfather and not another reason unrelated to sexual abuse.
Release the evidence anyway, it would be beyond stupid to let a (would-be?) sex offender like him who is in a position of power to get away. It only gives him another chance to strike at Umimi because Riku wouldn't always be there for her, would he? Better have him locked away for a few years of peace of mind than have him be walking free terrorizing this girl (or other girls, for that matter — would a creep like that stop at just targeting Umimi?).
Y'know what, now I think it'd be a good idea if he and Umimi dated at least for a while. He already went out of his way to risk his life to protect her, why stop there? Lines have been crossed, he made a choice in her favor, and now it's too late for Riku to simply back out of everything now and find new friends. Forget about Shizuku and Takt's little plots and mind games, shit got real with Umimi here.
Weird that Umimi's touching herself at the end after almost getting raped, but hey, if that's what it takes for her to calm down and sleep better after being saved by her knight in shining armor, so be it, I guess...
Taking the story in this direction really opened a can of worms for the readers, didn't it? Now some more bets are off when it comes to Shizuku's secret because we now know for a fact the author can and will go this route if they wanted to.