Just an update that my accelerator payment arrived to btc.com today and an hour after that the Bitcoins were deposited to my Binance wallet.
As I calculated, the fee should have been minimum 400 Satoshis per KB, while BTCMarkets only added 61. Interestingly, they added a comment about this on their Facebook page:
At this stage we have a system that hard codes the withdrawal fee but we intend to have a user configurable fee in the future to help during congested periods.
Anyway, thank you for your help. The accelerator was the quickest solution.