Yes, I would think this is possible or at least a lite version of it is. Between the sequencer, the cine camera actor, and the materials editor, you should be able to do something approximating this. The sequencer would be the key to it all. For a character talking to the camera, I would imagine something close to the comedy club template Epic already has. 3D animations would be handled by the sequencer and could also be controlled by the sequencer. I would just start with doing one concept at a time. The good thing is that UEFN and UE are getting closer and closer all the time. The sequencer, materials, and niagara are all pretty much the same right now, so if you search for Unreal Engine tutorials on what you have in mind, you should be able to translate them into UEFN. When you say alpha channel, the first thing that comes to mind is using materials, so it would be possible to use materials to create alphas and transparencies. Just my thoughts.
I also think there are creative ways you can get from one scene to another and this idea is fascinating to me. In the game, you can create Duplicate Universes and then cut between the two, so you can use this to get creative with how you can make things happen in the game. This video illustrates some of that: