Which tells me that Neville had left everything to Jeremy to relay his message to the police. Nothing sinister there except that if only Jeremy had remembered to think about his own part in all this.
Lookout, there are certain facts which are undeniable. One of them being however much you believe in your naive and lazy boy's innocence, his behaviour, after Nevill's alleged call, given it's OBVIOUS urgency and despite you saying he didn't spell that out in words of one syllable, was wholly and totally inappropriate.
As you're now claiming that he was relying on Jeremy to call them, I feel as if I maybe sensing a bit of a wobble in your, once, firmly held belief in Nevill's alleged call to the police. This being so, he won't have failed, in his alleged call to him, to convey to Jeremy, the urgency of the situation, making what Jeremy chose to do next even more inappropriate.
It occurs to me, that unless piggy wigs develop wings and something new and outstanding to prove his innocence is discovered, all you're going to be left with is an intuitive belief in his innocence. I think it would help enormously if you were to write everything down in a factual way -including how you believe Sheila accomplished the massacre- and bring all the pieces together. I won't pretend it isn't going to be risky because if you do it honestly, you may just discover that Sheila couldn't have done it. However, if you want to PROVE that Jeremy is innocent rather than just repeating it as a mantra, it could be a risk worth taking.