Seems like if you specify a parent to add the new nodes into, the parent node gets somehow mangled during the data serialization bit and is not usable anymore. This results in the newly loaded child nodes not getting rendered, among other things. I tried to fix it myself, but it's a scary recursive function
If it states that these root node attributes must be present in each .irr file, then everything is all right. If, on the other hand, it doesn't say so, then yes, some defaults should probably be used. I'm guessing the problem here was that it couldn't find those attributes in the scene file but tried to deserialize them onto my empty scene node anyway.