Mapped and duplicated content controls seem sensible to me too. You could use
https://www.gmayor.com/insert_content_control_addin.htm to aid the insertion of mapped controls, then all you have to do is copy and paste the duplicate control to the other location. Whatever you type in one is repeated in the other(s).
If you lock the controls against deletion, there is no need to protect the form. Protection for forms is a very weak form of protection. If people want to screw up your form, they will.