Are you talking about law/legal in general or specific in Russia? Besides, there's another concern whether OP could be remain anonymous if OP decide to sue the exchange.
Laws in general, I mentioned the EU because a wannabe lawyer here thought he has a clue how things work, but AML rules are basically the same everywhere and in every country, Russia in a member of the FATF same regulation when it comes to this should apply. The main thing here is that exchange X, considers those funds to be illegal, coming from a theft, anything else, both those courses of actions are illegal:
- seizing the funds and releasing them on a simple ID check, personally I don't believe they have a license to even deal with personal checks
- seizing the funds without informing the cybercrime or equivalent police branch and not giving custody of those assets under the mentioned law enforcement
The thing those exchanges are doing is simply random scamming people and getting those funds for themselves, since as you mentioned, it's quite hard to think that most of them would file complaints about smaller sums, and if they do all the exchange has to do is send him his funds and grab another victim.
That leaving aside that I am not very clear on the basis of what legal vacuum mixers operate, because in normal conditions a company that provides services to Italian citizens, should charge VAT and pay it to the government of Italy, which I doubt very much that the mixers do.
Financial transactions in which the ownership of funds in question is changed through the transfer of funds that do not include any other services are excluded from VAT. Chipmixer would simply act like those coin machines in which you dump all coins and you get a banknote or the other way round.