[chounokoe]

The problem I have with earthquakes and landslides is, that it is somehow too random to be actually true. It would be like a slap in the face and totally impossible to foresee. Earthquakes sometimes happen on an almost daily basis in Japan at least once a months there is one that is strong enough to be felt, typhoons happen on a not so heavy but also almost regular basis. Something like that being the case feels more like 'Whoops, oh yeah this time it got them'...sure it's possible but it would lead the investigation-process involving everything that happened on the island ad absurdum.

What I could accept as possible would be for example, arson. Someone set the mansion (and possibly the guesthouse, too) on fire after the incident and destroyed all possible evidence that was left...that would also explain why Eva had to be relocated to Kuwadorian for that night.
Back in the 80's, after a typhoon, on an island that was supposedly closed off from any civilization during those days, the forces may deemed that as an unfortunate accident.

We still also don't know what exactly was considered unfortunate and accidental about what they happened to find on Rokkenjima. On the one hand it is said they searched and found bodyparts and people went on calling it a serial killing, yet it was closed as an unfortunate incident...something is surely not quite right, but Ryukishi may be playing with us, making us expect something like the orchestrated Hinamizawa disaster.
How come the relatives of those killed on Rokkenjima never wanted to find out what happened? It's still odd that Ange is the only one who ever wants to find something about the actual events, and even she only after meeting Bernkastel.

What would truly help would for example be Eva's confession after she was found on the island. She was the only source of information left...and possibly she really implied something to the police that led them away from actual clues.

About the 'bombs beneath Rokkenjima' theory, surely possible...but it was said that you can get towards the answer with what EP1-4 gives you and there is surely not enough hard evidence pointing towards a military base, or any other facility harboring explosives, being located on Rokkenjima.
While Germany and Japan had a relation during WWII they fought individual battles and never really involved each other in the military regions of the other, and while I find it perfectly ample to believe that Beatrice might have been a woman of German descent based on her features, her love for English seems to imply either American or English descent.
Then again how would the bombs be set off? There is the accident through a small earthquake, but that is nonsense since there have been enough earthquakes in that area (alone this summer there were around 10 in closer proximity of where Rokkenjima would be located)...that would again require to much 'chance' and would be an even bigger miracle than Erika reaching the island.

Yes there is still the fact of Battler being alone, yet being killed...which is the big paradoxon which is leading us to the whole discussion.
But if it had to be a bomb it had to be prepared for a certain radius and a certain time to explode. Yet if the culprit was able to somehow avoid being present and yet being able to kill Nanjo in Episode 3 or Natsuhi in Episode 1, why would he bring him/herself in immediate danger of being killed in 'the explosion'? The problem here is that there would be no logic for that action and therefore it seems pretty much wrong compared to the otherwise very orchestrated crimes.

Arson may be the only 'accident' that would prove itself not totally counterproductive or too accidental to be true.