Update

After a couple of sleepless nights, where R. has been seriously thinking he may have gone crazy and set the place on fire, and i effectively have convinced him he had (gone crazy -lol-) we have asked the expert to re-open the investigation.
There are several things that have simply not been answered/looked into.

For instance, we had been smelling a strong petrol/gaz-like smell that whole evening and so had several of our neighbors and ppl we met in the cafe where we stayed after the fire.
What was that and how did it influence our situation?

R. had just finished painting our outdoor woodwork and couldn't those things have influenced anything (they were kept in the cellar too)?

The expert said although all of us declared we had heard a bang/explosion no evidence was found of this. But when R. went into the cellar the next morning he for instance saw the fridge totally out of place and simply blown away.

I hope the insurance company (who's paying for the investigation) is honoring our request and keep you posted.