Core.FailureObject is the object the game creates when it fails to create any object. Those can be deleted.
Core.Null is used as the script class used for temporary items usually used in animations. Those can be deleted.
The forwarders appear to be a wrapper for objects that are to be parented to another object to make them pickup the base objects interactions. It's used on things like the sculpting station objects, robot workbench objects and so on. It looks like a convention started with Ambitions then abandoned as it's not used in anything else. I wouldn't flush those unless you're sure you aren't flushing something in use which is difficult to know.
SharedFamilyInventory are the objects in your family inventory.

As for the script error, it can be ignored. Tagger was just trying to run while the game was shutting down and it got barked at.

Thank you! I hadn't notice anything going wrong, but I had been getting script errors, so I thought I'd check. So 26 is the number of items in family inventory, not the number of inventories? That makes sense then.