I got an answer, so we can't prove and should just rely on some anonymous person running mixer who can simply not answer for few days if something went wrong with service. If they have enough money, why support is so poor? Hard to hire few ppl? Lol.
I don't have any 'magical' suggestion, project is not opensource or so, it's business and problem should be solved by creators of service, not by me or community, i'm not getting money from it or any profit. I just pointed to obvious problem which allows mixer operator do whatever he wants w/o being catched.
And you already agreed that the problem exists, good that they have a lot of trust here, but it's still coin laundering anonymous service running by anonymous person, don't be naive, any personal problems (family problems or so) can force him to scam some deposit and you'll not able to prove it, anything can happen and customer should be protected from it, or, at least, prove that they were scammed, with current service site they can't do it.
Inflows and outflows of bitcoins, estimate from a blockchain analysis company called Clain:
If their estimate is correct, at a given moment Chipmixer often holds hundreds of bitcoins on behalf of its users.
They can also afford to spend about $60k worth of BTC per month on their signature campaign, not hiring more customer support would be more an indication of them having trouble finding a person they could trust, than lacking money.
Any family problems to be solved with money also shouldn't be an issue, they must be making a lot of money from voluntary fees alone with their volume.