In general, each object will match a set of objectClasses and the allowed attributes are the union of those allowed for each class. I agree it's powerful and problematic. I think I've seen something about LDAP implementations requiring single inheritance. I'd say yes. No, I didn't mean to imply that aux classes can be standalone.
Every object has to implement at least one structural objectClass.
The last one above fails because it has no structural class inheriting from GridResource. Contrast this with the last example in the OK section above. Each entity must include one of the defined abstract object classes. This is unclear to me. Perhaps An abstract class is used to define a base set of attributes that can be incorporated into other classes.