Courts usually order an amount of statuory interest. Typically 12%, although it might vary by your state. Read the court order from your case and it should tell you.
The garnish must be issued by a court (unless we're talking about a Federal Student Loan).
Read the court order; it will say how much is to be garnished and if it is to be recalculated in anyway. If what the lawyer wants is not established in the court order, he's out of luck.
I don't think so. The court order specifies by law what is to be collected, how much can be collected, if interest was part of the settlement, etc.
However, you could still be charged attorney's fees if your creditor had to pay them.
You'll actually have to read the court order.
Yes, a creditor can collect interest on judgments. How much depends on state law. Interest rates on judgments vary from state to state.