Last time I checked a SDK stood for Software Developers Kit, which is what the download is, for it provides samples on how the menu can be used in different scenerios and is indended to be downloaded by software developers.  Being an open-source project, it seems natural (to me) that this kit would contain the entire source code as well, as opposed to including only the binaries in it and providing a separate download.  This way a developer could modify the menu code as well as the samples.

As far as testing something other than solpart I believe you are making things much more difficult than necessary.  The default skins included in DNN contain samples of other controls without the need to concern yourself with usercontrols (skin objects).  However, if you still wish to persue using Nav.ascx for another control I can answer any specific questions you have to get that approach to work.