Here are the answers to your questions.
The interest is required to be paid periodically on the used amount because it is how margin works
It is because of the margin limits that restrict you from buying
I have never used IBKR, so I am not in a proper position to answer it precisely. As you said it is not reflecting in the statement, I believe it will take a day or two to reflect, you can wait for a maximum of three days and then get in touch with the customer support team if it is not resolved.
