Hello,
1) We did not remove your changed theme. We just renamed it http://prntscr.com/vwratf and uploaded source files as the parent theme to make sure that your changes do not affect the add to cart option.
2) If you made changes in that theme then move them all to the child theme. Use child theme functions.php to place changed functions. It works fine if you place them in the correct way. If you need to change styles you can use child theme style.css to add your custom styles and overwrite parent theme styles.
Your custom is ok once I purged the CDN cache http://prntscr.com/vwrc6x
3) What other changes did you make in child theme files? Move the changed files to child theme, please, and then reply back and our developers will start to work on the q-ty issue.
Regards