The two exchangers you mentioned, imExchanger.pro and Openchange.cash, don’t have their own liquidity. They operate by using WhiteBIT as their "liquidity provider."
1. When you send crypto, it likely goes directly to a WhiteBIT deposit address.
2. They then process payouts from WhiteBIT and take their commission on top.
So it’s very possible that your funds were frozen by WhiteBIT, not necessarily stolen outright by the exchanger.
I’m not defending them, just pointing out how these setups typically work. It might be worth contacting WhiteBIT support directly with full transaction details and any screenshots or communication logs you have.
The bottom line is that the exchangers have smoothly distanced themselves from the whole problem and do not offer any help to the OP in solving it. So they are only there when they need to take a percentage of the order.
Why would anyone use them later? It's simpler to go directly to Whitebit, probably the client will have more chances.
Based on various previous similar reports of this type, it is noticeable that most exchangers on Bestchange are Whitebit resellers.