Do you have order request (or statement of work) signed from her or her business alias? Did you take security deposit before starting to fill her order?
If you do, can't you take legal action?
If you do not, then you need to reconsider the sentences below:
"In our last meeting, she questioned my communication skills, so I wrote her such an email, that she might need to go back to school to learn English."---communication doesn't always imply your command of certain language. If you give this statement of yours a second thought, you might find your weakness. Hint: she is German.