I think she didn't want to come off as impatient. There are some artists out there who will snap at costumers if they tell them the commission is taking too long, and she probably didn't know if you were one of those people or not. She probably didn't want to risk it.

Unfortunately I don't know anything about how to get a subpoena. Sorry.

And something I forgot to mention initially. Threatening to take the OP to court is a little uncalled for at this point. You don't have proof except your word right now, and there's always a chance to discuss this calmly and reasonably with the OP before resorting to court. Like other people have said, explain your side of the story in a calm manner, even if you don't have proof just yet. That's the best thing to do right now.