It's marked in the thread you referenced that Marcus only excepts US based customers.
I assume it is best to close your account with them and let them transfer the remaining balance to your European bank.
Not sure how you can achieve that now, since you probably lied/faked to them before that you were still living in the USA, and your account is flagged now.
I also don't believe filing will complaints will help, since you seem to be at fault here, and they have to lock your account for compliance and security reasons.
When you unlock your account, I strongly also suggest that you use your local bank to exchange the currency (or Marcus and make a € transfer). IB currency exchange is meant for trading purposes, not for exchanging currencies:
See here: https://www.reddit.com/r/interactivebrokers/comments/ywj5hw/...
What do you think will happen when a transfer in $ comes into a newly opened account, you convert it to €, and then cash out to a European € account. All alarm flags will probably go off, as this is classic money laundering. I would definitely ask their ok in advance and provide them documentation on where the money comes from, before trying to exchange money through their platform.
Depending on how large your transfer is, any european bank will flag the transfer and you have to provide proof of the source of wealth. It's best to provide that documentation beforehand, because otherwise your funds will be blocked for a few days too before you can access them.
Also, in case you have signed a "compromis de vente" without a suspension clause (e.g. that the contract is void if the bank doesn't finance the acquisition, or something similar), then yes you are on the hook for the 10% in case you don't go to the notary in time to buy the house/appartment.
Good luck!