Are the headers unlinked? Otherwise, you have the same text box in both
sections, and that is probably what is causing the error message.
I am not sure what you mean by "story area"; usually, the space for ordinary text
is called the "main body" and other areas, such as headers and footers, are
referred to as "story ranges" (at least that is the terminology in Word VBA).
Document protection should prevent users from changing content. Whenever you
want to enforce changes, using a macro that unprotects and reprotects the
document should work. In fact, that is the way it is done in a macro that enables
spell-checking in a protected document:
Microsoft Word MVP