Automatic export to .csv when saving .numbers
I think you can just bypass using myfile.numbers all together. Instead of saving myfile.numbers (step 3), just export myfile.numbers as a csv, and quit Numbers without saving. 

This way, your workflow is:

1) Open myfile.csv with Numbers

2) Edit myfile.csv in Numbers

3) Export the current file as myfile.csv, which overwrites myfile.csv with a new version.