Is it feasible to build a central "Text editor settings" control module?
Suggestions
If "Open Sans" is not in the list, then it must be the default. If its the default then removing the formatting would return it to the default.
#3 a search for "tinymce adjust avaialbe font sizes" turned up this:
https://stackoverflow.com/questions/20183796/tinymce-4-how-change-font-size-styles
which suggests adding this to your form_editor.tpl file
fontsize_formats: "8pt 9pt 10pt 11pt 12pt 26pt 36pt",