Let assume my base currency is EUR and that I have deposited 200K on margin account. If I understand correctly if order SELL EUR.USD 200k is executed, then the currency conversion will happen. If additional I invoke another SELL EUR.USD 200k, then I would be short 200K and IB would count interest on 200k in currency EUR, right? If I don't have available USD currency and I buy US stocks, then interest would be billed in USD currency, right? So if I want the margin to be in EUR currency and to use it to buy US stock, I'd need to SELL EUR.USD (to be short EUR) and than with converted USD to buy US stocks, is it correct?
Correct. Correct. Correct. In addition to your third question: you could first buy the US stock and see how negative your USD cash position becomes. Then you sell sufficient EUR to flatten out this negative USD cash position.