Add a relationship from a to C. This relationship will always stay empty, in the description of the field leave a note tgats to make a view and to keep it empty. You can also add a relation filter for where public ID is -1, this the drop-down will always be empty.
Then in the entity view, go to this new relation, and press on “Filter” and the it will say “Show C from a” press on C, and it will give you the option to show c from B from a. Now it will act like a lookup but with the ability to create, and show as any layout. Down side is that it makes a fake relationship, but until this: Add Views to Entities makes systems or this: Add a view of "Collection" entities within another entity that are not related, just for reference
I think this is the only option.
Note that in table view this fake relationship should be off, and the lookup should be on. As this complex filtering only works in entity view and not in a compact list unfortunately.
Lmk if it works!