[Essential] means they cannot be killed [...] and when their health bar fully depletes, they will merely fall into a state of recovery for a short time [...].
A key criterion of my question is the fact that essential NPCs can still be hurt, allowing one to grind skills that require inflicting damage on them. A prime example is Ralof:
Before you escape Helgen, Ralof will not retaliate if you attack him.
If they are only essential or will not attack you at a certain point, when?
Why do I ask? Because instead of asking "who is a good target on which to grind Destruction" or somesuch and getting answers mentioning just one (or few) NPCs, it'd be more useful to get a complete list.