Sounds like OpenChange need to come clean here and tell OP which third party service is currently holding his coins ransom.
Did anyone ever saw any of their team members in bitcointalk forum?I doubt they will register here and explain anything, but there is still hope to get this information via BestChange forum members.
I would be asking for the results of the audit so he can independently verify how they have decided that mixed coins are "100% stolen", since that is clearly and provably incorrect.
I can't wait to see this ''audit'' 
How authentic these reviews are? I feel negatives were not displayed but only the positive reviews are there.
This is standard practice with most websites and services, they only show positive feedback with questionable history.For example, I know ledger model X hardware wallet had many complains from customers (battery, firmware issues, etc), but if you check their reviews page it has almost perfect score:
https://shop.ledger.com/products/ledger-nano-x?view=reviews