World Serpent's Grasp is defined in Heroes of the Forgotten Kingdoms on page 321. It says:
Whenever you hit a slowed or immobilized target with an attack, you can knock it prone.
This has three conditions:
- You make an attack
- You hit with the attack
- The creature you hit was slowed or immobilized
Condition one is not as trivial as it sounds, as you need to determine whether what you just did was an attack. There is already a question and answer for figuring this out. For the most part, if it has an Attack line, then it's an attack. The Mage's Magic Missile attack, defined in Heroes of the Fallen Lands, is an example of an attack without an Attack line.
Condition two is a little easier, but also has difficulties. I can't find this explicitly written out anywhere, but based on how everything else works I would say that you haven't hit unless the power has a Hit line. That being said, according to the Player's Handbook, on page 57:
Every power that requires an attack roll includes a “Hit” entry, which explains what happens when an attack roll succeeds.
Magic Missile has no Hit line, since a Hit effect is dependent upon a successful attack and it has no Attack line. The damage for this power is expressed in the Effect line. So, I'm not sure if doing damage with Magic Missle technically counts as a 'hit' or not with the rules-as-written, but as DM I would certainly count it.
Condition three is self-explanatory.
You haven't cited specific powers for most of your example cases, which makes a definitive answer impossible. However, assuming that nothing is too out of ordinary, then the answer to all of your examples is yes.
This includes the Iron Vanguard feature, which would definitely trigger as you have definitely knocked a creature prone. To be clear, this feature does not mention an attack or a hit, so even if the way in which you knocked the creature prone was not an attack, did not hit, and did no damage, it would still trigger.