Starting from the beginning:
You can't use "Asgard Must Fall" as part of multiattack. Double power actions cannot be given as free actions.

The effects of the action take place immediately. You can't give them an action to use the power, do something else, and then decide to use it.

There is no falling damage, because the characters are not being knocked back off elevated terrain.

If there is Elevated terrain that has blocking terrain on it, does that elevated blocking terrain become grounded hindering terrain too?

Now that's an interesting question. Since it doesn't say what to do about blocking, I'd rule it remains blocking but is now grounded (and surrounded by Hindering).

Because's it only ATOP The Elevated?

I'd assume all objects and misc. would remain after the Shrine has "Fallen"

Well I wasnt sure how to rule it last night, in the end we agreed the blocking would become hindering.
All it says is all elevated becomes grounded hindering. It was elevated and it cant be hindering terrain if it's blocking terrain.

But honestly it makes sense the ability is meant to destroy stuff, so why wouldn't all the blocking become rubble? Although walls would remain intact.

This is how it was ruled at today's tourney as well. Curios if that was correct.

ASGARD MUST FALL: Once per game, give Sentry and Void a double power action. For the rest of the game even if this power is lost elevated terrain is considered grounded hindering terrain. Deal Sentry and Void and each character occupying elevated terrain 1 unavoidable damage.

Nothing says that blocking terrain is somehow exempt. Simply put, if it was elevated terrain (whether clear, hindering, water, or blocking elevated terrain), it is now grounded hindering terrain. No more and no less.