1) Yes, you have it correct
2) The overlord can place burn, web or stun tokens or use the knockback effect whether the hero takes any wounds or not as long as at least one heart has been rolled on the dice, limited to one per attack not per heart rolled.
Imagine that you are walking into an ordinary old cupboard, there are spiders living in this cupboard and cobwebs are strung across the room. Walking into them doesn't harm you but it does get cobwebs over you and would take you time to remove them.
In Descent the spiders are huge and so are the webs, thus you can be tangled in them without taking any damage. The same applies to all the other effects, your clothes can be on fire without it actually being hot enough to burn you immediately but if they catch light... etc etc.
Hope this clears up your question.
I could be mistaken, but isn't there a proviso that if the Overlord rolls a "X" then the attack automatically misses and it doesn't matter how many Hearts are displayed on the other dice.