If they are an employee under legal agreements, than a firm retains copyright. Yet if the render node is another companies property, than it is highly advisable to talk with your IP lawyers to figure out the jurisdictional copyright rules in your country.

'Rendering an image is not "creating a work“ in the copyright sense.'

Perhaps your super awesome precedent case against BBC, Sony, and or Disney will end in your favor. It is not 0% chance, but pretty close from what the IP firms have advised.

'If the Blender scene has been modelled by someone else, then yes, that someone owns the copyright'

That is just not true in most places, and including assets/compositions from libraries you "found" does not assign copyrights to use said work. Even if some con artist on the Unity store sold you "rights", there is no guarantee it was theirs to assign.

Please consider talking with an IP lawyer in your jurisdiction, as your ludicrous notions of ownership could end up a liability. lol Best of luck =3