I'll go with the first part of DL's answer: "severe market turmoil, the US dollar would lose it's world reserve currency status, extreme hyperinflation (think Zimbabwe)"
Heck, I think all that will happen even if the vault doors are never opened again.

I don't believe we'll see mega-states or new fiat currencies. More likely succession, a lot of smaller nation states, and some sort of PM-backed cyber-crpto-currency. Think bitcoin but written by someone who knows economics and the theory of money and credit. Eventually I expect phyles or something similar to destroy nation states (please!) but I don't expect to live that long.
Question 2: this one is rather unlikely. We know where there is that much gold, and more, but can't reach it. It's only a few thousand miles away, in the earth's core. Go get some, I'll wait.
But even if a gold asteroid were captured and brought to earth and produced the market price decline to 20% of today's value, nothing beyond beating up PM holders would happen.
It doesn't matter how much money is available; what matters is that it is hard to create more of it. Fiat is so dangerous and destructive precisely because it is so easy to make.
Do this thought experiment: Tonight we all go to sleep. Tomorrow we wake up and the amount of money in our bank accounts has doubled. Are we all richer? Of course not. Prices will also double, more or less, but not smoothly and not instantly. The early risers who figure out was has happened first will clear the store shelves, then sell it back at whatever the new market will bear.
If you hold PMs, you get beaten. It's a fact of life in this world. So literally nothing new would happen.
Question 3: no country is currently on the gold standard. Switzerland flirts with the idea, a referendum would have strongly linked the Swiss franc and gold failed recently.
China is quietly building its gold reserves, and could destroy the US and most Western economies by linking their currency to gold. Destroying your customers is bad for business, so I don't expect China to do that soon.
Several middle Eastern nations talk of gold currency, in part to escape the tyranny of the US petrodollar. Since they have only one thing to sell, and sell to the whole world while their share of US market falls, they aren't as constrained as China. There have been several proposals to mint gold dinars, and some reports that ISIS uses gold coins.
I expect a smart, Austrian-aware entrepreneur to develop a Bitcoin killer backed by PMs. Nation states hate gold for good reasons, and the consumer market will love a good gold currency for the same reasons.
Peace,
Silver