The Fed has no choice but to hold them to term. The Fed's whole game is to continue keeping rates at zero by buying everything under the sun to the tune of $85 billion! per month. Selling, or even slowing the rate of purchasing, would do the opposite and raise rates.
They have choices. They have complete flexibility. They are under no pressure to do anything that isn't explicitly their goals. They want to control inflation and increase employment. If the economy starts to heat up and inflation starts to rise above their 2% target they will then have the option of selling bonds or raising the federal funds rate, the latter of which is more likely.
I'm not really sure what you're trying to get at with the last comment. He doesn't know the exact number off the top of his head. The exact amount remitted last year to Treasury is $88.4 billion.
They are under extreme pressure to continue purchasing treasury bills from the government and mortgage backed securities and derivatives from banks. Even hinting that they'll start tapering these purchases has caused the equity markets to drop and rates to rise. Actually tapering will in short order throw the country into a recession. I'd be hard pressed to call that "complete flexibility."
