Does the $form['element']['#disabled'] = TRUE guarantee that the user cannot edit the field or is it just a UI setting
If you disable field in backend, the field value won't be actually changed even if you change it in HTML or by JS.
You can test it:
 - disable element in form alter
 - open element in Chrome inspector (or other) and remove disabled attribute
 - change value in field and save the form
 - changed value will not be saved