BuddyPress.org

I think you’re over-thinking this a little bit.

bp-default assumes that it is the “home” template, and nothing more. It assumes that user blogs will use other templates that you either create yourself, supply to them, or that they can install via the Appearance item in the site admin.

The only way to trick this into working is by putting…

define( 'BP_ENABLE_MULTIBLOG', true )

…either in wp-config.php or /wp-content/plugins/bp-custom.php. This still probably isn’t really what you’re trying to do, but it will get you where you want to be without modifying the core templates. If you’re using a custom template, then it’s as easy as modifying the header.php to always link back to the root blog components and pages that you want it to.