Is it possible to hand over from Siri to the App providing the App Intent (i.e. open the app from a Siri as result of an action)? For example, this could be handy when I actually want to show some UI in Response to a customer's prompt, like opening a Photo Album.
Yes. There are variations on the .result() you can return that allow you to provide an opens intent https://developer.apple.com/documentation/appintents/intentresult/result(opensintent:)-6l7s5
In Shortcuts, the action will have a "Open When Run" toggle which will enable or disable running the provided opens intent.
Siri may decide when to run the opens intent based on the current mode