I can understand the difficulties you are having with your textbook. It is often difficult when a teacher is not able to choose the textbook and they have to work with what is provided. We often think that textbooks need improving and this is true for most books. No textbook is perfect for all groups of students.
I don’t think that writing your own textbook is the solution. This is a difficult task and takes a long time. It is very different to writing one-off activities because of the aspects of sequencing, revision and covering a variety of skills and language points. I think the course will be over before you finish writing the book. I don’t think you will find a free textbook on the web because of the amount of work it involves. If I am wrong and you find one, I would like to hear about it.
However, I do think you will find materials to supplement the textbook and this is a good place to start. You say that the language presented is not idiomatic; so you can introduce activities to practise idiomatic language. You will find these fairly easily on internet. You can also explore ways to teach the grammar. Try to find ways to explain the grammar in English – you can do this with mime, with pictures, with real situations, etc…. If you would like more ideas about this, you can write in again. Think carefully about how you work with the Burmese teacher. Try to plan ahead with her. By doing that, if you come across something she doesn’t know, you can explain it to her. This will be better than having her translate on the spot.
I am glad to see that you are thinking about the legal aspects of photocopying books. Many authors make less money than they could because people photocopy instead of buying books.