Hmm Buff do you think Yashiro locked his car? I'm not entirely sure he would. It's a dumper and there's nothing suspicious, he was also giving rides to those other schoolgirls. I don't see any problem with the locks being left open. And by now Kenya seems like the type bright enough to be narrowing his own list of suspects, he's definitely figured out Satoru's game as well so seeing that Yashiro is the likeliest of the bunch and the only one present, he could've been keeping an eye on the gym and crept in when he saw Yashiro and Satoru coming out.

But definitely agreed on that Yuuki would be the best symbolic savior at this point. I just don't know where he'd come from. Other than the kid group, Yuuki is the only bigger character missing from the final flashes of the episode. If we assume that was intentional, he's probably going to be it.

The theory of Kenya using Yuuki to get out there has the trouble of that we've never seen Kenya meet Yuuki (not that I remember) and I haven't seen Satoru mention Yuuki to Kenya either, so a smart kid would probably be avoiding Yuuki rather than enlisting his help.

If anything, I'd think Yuuki was just there and noticed something off with Satoru, or was maybe watching Misato, and followed them out to the boonies. Difficulty with that is what would scrawny Yuuki do against a killer watching the car sink?