I doubt you will be able to find a good way to prevent change to the header
unless you use preprinted paper, but as long as you really mean "unchanged"
then I would probably create the header as an image, i.e. the only change a
user can /inadvertently/ make is to delete or perhaps move/resize the
header.

I would probably set up the footer fields as { DOCPROPERTY } fields using
custom properties so that all the site/user really has to do is do a one-off
modification of the property values using the built-in dialog box.

Just my 2c worth.