Hi DKnox,
To answer your first question, Culling does not automatically split up objects or remove parts that are unseen from being rendered if the object is a single actor. For instance, if you have a building with multiple interior walls and some additional assets that are all part of the same static mesh, these parts will all still be rendered even though they are not visible to the camera. You would want to have these be modular assets and break the building into smaller chunks to keep them from rendering when not visible.
At the moment, there isn’t a large bit of information available for visibility culling with UE4 and this is a concern that was recently discussed with our learning resources team. There are plans to get this information written up in an easy to follow page. This is also something I’ve been putting together for my personal site that will likely get repurposed for a documentation page starting point. The page I’ve started on is still a little rough and isn’t publicly linked on my site yet but you can read through it here for more information.
That page should contain the answers you’re seeking. Should you have additional questions feel free to post back here. 
Tim