Not an official mandate mind
He's talking about risks from other crises and says that the Fed really has three mandates. That's pretty much the mandate of every central bank, written in or not.
The fact they didn't have their eyes open to that 8/9 odd years ago is why they ended up in the car crash.
A stock market crash wouldn't have the same effect as a housing market crash
The balance sheet will be reduced in the not too distant future but it will probably stay higher than previously due to the nature of the economy now
The Fed will reduce the balance sheet when the economy is ready
Q&A with Kashkari