1) Generally speaking in Europe, after more than 5000€ in transactions the last 30 calendar days, banks (and related services) need to check out if there is nothing unusual. But in fact, it's automated most of the time, and your account(s) aren't even checked if, for example, you send money from your account 1 to your account 2. To make it short, there is no money laundering risk if you send money to yourself.
At worst, they may ask you why you are sending or receiving so much money to this or that person. Or ask you about the source of funds. But you can say a little bit of anything
2) I don't know this service so can't comment about it. But the only way to get your funds back will be after a justice decision, which can take perhaps at least a year or more. Don't you have its prepaid card to try to get $ via ATM?.
3 )It is ambiguous, legally they can ask for the origin, but not necessarily to ask for concrete evidence (such as supporting documents). That's why I was saying that people can talk a little bit of nonsense. It's like you're getting money from your aunt for your birthday, what proof can they ask for? You see.
Can you advise another friendly platform with IBAN DE?
There is no DE, NL or BE Iban, it's a European standard but if you mean a DE company, take a look at Fidor and Bitwala.