

This is especially problematic as media […] gets duplicated to every receiving instance.
I’m not sure this is actually the case. The posts certainly do get duplicated to every instance, of course, but the media contained in them are always hosted by the originating instance, as far as I can tell. So once it is deleted from there, no one can access it anymore.
I think it’s an interesting idea though. Since you mentioned votes by members of the individual instances, I wonder how you would handle the different sizes of instances. I’m the only user of my instance, for example. I probably wouldn’t be able to be judge then, right?
You can change your default listing type in the user preferences.