yeah, legally speaking it simply not a "crime" in the legal sense, unless you can prove that he INTENTIONALLY lost the coins or that he INTENDED to "steal" them. When he says explicitally that this was NOT intended (as he does) it is basically impossible to prove the opposite.
Even though he had no right to gamble with your coins, you cannot assume from that, that he "stole" them or did this in any way intentionally.
I mean you have the right to be reimbursed for your loss, but it is not a crime. The ONLY thing you can do is to sue him for the damage he caused to you and that is
a) tricky to prove
b) useless for the above metioned reasons
The government and all the crappy police institutions plus courts etc. are unfortunately NOT here to help you, they don't give a shit, they are just there for their own sake of being paid by the tax payer.
PS: 15 min at the police? Last time it took me already 45 min queueing and after that 1 hour just to explain what a bitcoin is and why it has value, lol
Are you some kind of legal expert? Not all crimes require intent. The proof of negligence is often enough, for example negligent homicide. The fact is that M4v3R took (knowingly and willingly) something that didn't belong to him, used it for his own intents and purposes, and subsequently lost it. Whether or not that is a crime is best left to the judicial system to decide.
You don't actually need to go into the details on bitcoins and their value, like you don't need to explain why tokens of fiat money printed on paper have some value. You just need to explain that bitcoins are a form of digital goods.