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Hiya, no it's fine to keep the global nav on all pages... you should do anyway. What I mean is, trim the number of links in that global nav down by placing some of them only within their category areas instead, in a secondary menu maybe. But... you can still use flyouts in the main menu so that the menu items are listed and then a hover-over of a category brings out more items, and again for a sub-category, etc... in which you could use a technology which isn't crawl-able/followed. That way users can still access all menu items from the main nav but Google will only see the top level links, with further links not followed (without using NoFollow) and bypassed. Not sure the best way to do it though since JS and Ajax are by all accounts a lot more crawl-able now.

It's best to have the keywords that you're targeting as close to domain as possible although it wouldn't matter when you have just two short folder names such as /letters/abc. Good luck!

As above example/abc rewrites to example/abc.html example/abc.html redirects to example/abc and all internal links link to example/abc