To answer your question in a round about way, the internee prisoners of Scholss Itter were almost entirely VIPs. People held there because they were politically important, and to be used as a last minute bargaining chip with the Allies. To say that they were forced to do hard labor is an embellishment, especially compared to say what millions of others had to endure in various Concentration Camps working in various factories, or the forced labor used in the Todt Organization's Autobahn's works projects. However, as you would know from reading that book the SS wanted to execute all of those internee VIPs. So there is that (hence why US Forces, and Austrian Heer [Army] Soldiers / Resistance fighters fought against the SS, which is why the book is called "The Last Battle").

Now back to your first part. Nazi Germany would have used the various conquered people to work in the factories, on farms, digging ditches etc... basically exactly like what happened during the war.

A lot of skilled laborers were actually paid... contrary to popular belief. For instance in his book "Berlin at War" by Roger Moorhouse, which describes the civilian life in Berlin during the war, many skilled laborer non-Germans moved to Berlin to work in the various factories and industries and were paid - although not too well.

What Germany would have done had they won the war is slight speculation. But most likely they would have continued using forced labor from conquered countries, paid skill laborers, and continue on their quest to conquer more territories and keep up the modernization of the German Wehrmacht.