How do I limit the number of keystrokes a user can add to a content control field? Say 5 keystrokes?

One thing you could do with a content control is to change its placeholder text. On the Developer tab, click the Design Mode button. Edit the text -- for example, to "5chrs" -- and then click Design Mode again to turn it off.

If you have to use legacy form fields, you can insert section breaks (probably the continuous type) to separate areas containing form fields from areas that need full editing/formatting. Click the Restrict Editing button. In the pane, check the box in the Editing Restrictions section and set the dropdown to 'Filling in forms'. Then click the 'Select sections' link below the dropdown, and uncheck the boxes for the full-editing sections. Start enforcing and supply a password if desired.

Some features will still be disabled even in the unprotected sections. While you can insert a picture from a file, you can't insert clip art, text boxes, shapes, SmartArt, bookmarks, cross-references, captions, footnotes, or endnotes. But bullets, numbering, and general formatting will work in those sections.

As for auto-populating, read http://gregmaxey.mvps.org/Repeating_Data.htm. Some of the methods there use macros, but most don't. For headers specifically, read http://wordfaqs.mvps.org/StyleRef.htm.