Forgive a newbie trying to help a fellow newbie.  In the absence of comments from the wizards I would suggest from my reading of the Zope book that you will find the issue has to do with the namespace that is at the top of the stack.  I don't think any acquisition happens by going down a tree.  Instead, the acquisition of index_html at the root put all of the root's objects into the stack and anything matching the target name is being executed from that context.  I suspect you need one of the namespace managing commands to put the context back at the 'here' level (lowest branch/leaf) so that a missing target in that context will be found by acquisition going up the tree instead of a match at root happening first.
 
For what its worth!  If this makes sense, you might post a question about the syntax to control the namespace and get more focused replies.  The answer could be as simple as a <dtml-with ...  filled in with here or the underscore namespace, but I'm over my head to guess further.
 
Your navigation plan sounds great.  Simple elegance!  Looking forward to seeing how you accomplish it.
 
If I can help further, yell!  I know how frustrating a no-reply on the maillist can be.
Regards,
Gary