What you're explaining is what Kentico is used for. A custom page type is by far the best way to go. Yes people COULD navigate to those pages but changes are very slim the will ever because they need to know the exact URL of that "page".
Secondly there should be no need to uncheck the "show in navigation" if you set your navigation webpart up properly it won't be an issue at all because it will already be filtered out.
Thirdly, there is a random document webpart which will do all the work for you if you're looking to randominize how your hero images are showing. Another option is to specify
NEWID() in your Order By property, which should get a random document