Hey,
your second code snippet never sets self._result.
Regarding loading vs. cloning documents: That I did it how I did it was no accident, but that does not necessarily mean that cloning things is bad. C4DAtom::GetClone is implemented in the different scene elements independently; i.e., BaseObject::GetClone, BaseDocument::GetClone, BaseTag::GetClone, etc. and C4DAtom::GetClone is just the empty implementation. The different implementations then rely on C4DAtom::CopyTo to copy data from the reference into a new instance of that scene element type. On top of that comes that Cinema 4D is largely Copy-on-Write these days, with data actually only being copied when you try to modify it. This all explains why BaseDocument::GetClone is so inexplicably performant, it just copies the metadata and only copies the real payload of the scene when it has to. On top of that comes that cloning from the same document from multiple threads in parallel could entail read-access violations (although somewhat unlikely).
On the other hand, our rendering pipeline does exactly the same, it clones documents for rendering (the cloning is however done on the main thread).
I personally would say what you are doing is probably okay, but I would not write such code as example code. I would have to spend more time on this to figure out if this is absolutely safe. Doing the actual cloning off-main-thread seems risky though (but is probably also fine in 99,9% of the cases).
Cheers,
Ferdinand