property is a property of Section
and the logic I sent you applies the headers and footers to your merged document.. Thus, setting the property to your template document has no effect over the merged document. What you can do is specify the property when iterating through the merged document's sections:
section.Headers.Default = header;
section.Footers.Default = footer;
section.FooterBottomMargin = 23;
When it comes to the width of the footer - that depends on the way your template document is created. From the look of the picture you attached I tried creating a table without borders in Landscape
orientation mode, which is set to auto fit
to the width of the pages. Applying that document as a template successfully resized the footer.
If, however, that does not work for you, please send us more detailed explanation of your requirements, so we can help you further.