[haguruma]

Originally Posted by Renall

I don't think circumstantial evidence would even make it that difficult to pin down to a servant, presumably female, who had been present on the island for a long period of time. Really, given the descriptions people would provide (Gohda wasn't the only one who saw it, I'd think) I'd probably narrow it down to either Jessica or Shannon.

Well yes, it's not even unlikely that this might have been one of the stronger leads that some might have followed up upon, but then there would have been no activity shown that would incriminate the 17 year old servant girl beyond a basic doubt of "she was after the money".
It was mentioned that the case against Eva was basically dropped because, even though she was considered a suspect in the beginning, they couldn't produce evidence of her planning anything out of the ordinary when heading to the last family conference.
The same could be said for Shannon, because there would be no evidence of her entertaining any moves outside Rokkenjima, except her holiday with George. I think even the bank account was installed by Genji for her. Had she really planned this as a way to get towards the money, she and George should have survived. Even more, the author of the message bottles seems to hold an intense hatred for Shannon and Kanon (no matter who they were) antagonizing them in a degree that's not easily explainable.

After all, if "Beatrice" was the culprit, and did commit a crime, but died doing so... well, case closed. The nutjob who wrote these message bottles did it, nobody to prosecute, everybody can go home.

The problem here is that Maria's Beatrice and the Beatrice of the message bottles are inherently different in there characterization and it is common knowledge that Kinzo spread the story about meeting "the witch Beatrice" since the 1950's apparently. The question why somebody who'd portray this witch as a sweet and kind friend to a girl, yet portray her as a murdering monstrosity in a story remains unsolved without further evidence.
Yes, you could end on the agreement that "a nutjob who actually believed in a witch came close to Maria and finally blew them all up", but several things just don't end up with that solution. Which is why I think it is not entirely unthinkable that they'd leave the case undisclosed.

Going by what I said above, about incriminating evidence, Kyrie is actually the one with the highest amount of "preparation" going into her departure towards Rokkenjima. She not only left her daughter at home, it might have even been possibly to produce evidence that the 3 siblings and their spouses were in a discussion before heading to Rokkenjima, the son of her husband suddenly returning after a 6 year absence. It is not that hard to understand why so many people in-universe seem to believe in at least a Kyrie culprit theory.

The ep8 manga also suggests the clock bomb trigger thing was known to some people who were Kinzo's business associates. It's certainly possible to dismiss it as just some rumor... unless you're the police and you're investigating an island that just exploded.

Yes, of course. But unless you find the person who actually built the trigger, wouldn't you be stuck with just witnesses testifying the existence of a probable cause of the explosion?

Does this prove criminal activity? No, but in and of itself it's certainly circumstantial evidence. Notably, you can make a very intriguing logic chain for an Eva culprit by arguing

Like you said, it is an intriguing argument, but not entirely unshakable.
The strongest argument against Eva culprit actually being what Okonogi also brought up, the death of Hideyoshi and George and the decline of Eva's mental state following their funeral. Yes, you could argue irrational acting in a moment of high stress, but this are all very hypothetical scenarios.

We don't actually know that the woman died. That information was lost with Genji and Rosa (or was it? How'd it get into Banquet?).

Well, the information in Banquet very likely came through Touya, who might have heard it from Rosa while on the island. Banquet though was only released like 1996 or 1997, so probably after the investigation was already put largely on ice, putting aside that "fanfiction" on the internet would have been an even more obscure thing in mid/late 90's than today.