With regards to the external link you could create a link node, hide it from the node list and then select it to show as tab from the add-on and link it to your external source. Unless I'm misunderstanding what your wanting to do.
Use link-forums. They are nodes that function to redirect to a URL that you specify inside of the link-forum. The NodesAsTabs addon can create nav tabs out of link-forums like any other node type. In that way you can use NodesAsTabs to create nav links to external URLs.
The NodesAsTabs addon is more functional and doesn't require template edits, but for simple external links in the navbar the addon may be more than you need.
edit - I see you also want nav links that go to threads in your forum. For that you really should use the NodesAsTabs addon. The addon supports tab selection which you would probably want for an internal link like a thread.