The biggest mistake he made was not telling his mom. He should open up to everything. The obvious weakness Satoru has in the current situation is his youth, which is why you need adult allies. His mom is the most obvious choice.

I mean, if he was ready to push that lady off the stairs (which he should have done), he could at least risk being labeled insane. If Kenya accompanied him to explain to his mother, it would've worked. The kid is more eloquent than Satoru at any age.

It's definitely not the killer meta-wise. It's either Yuuki or his mom, but that doesn't excuse his stupid planning.