It's a bit difficult to test an instance method at the class level. Once you've instantiated an object from a class, now you're debuggin' with objects.
Depends. There are systems where everything is an object. Including classes (and if you ask what's the class of the class object the answer is Metaclass).
In those systems, you technically always work on objects.
Well, technically you're still debugging the class, not the object, you're just using the instantiated object to do it. :)