Is it possible to sell those bonds back to the Fed right now or do they have to wait 10 or 30 years to do that?
Whe they sell those bonds, do they get dollars back?
Sell not redeem.
No, the buyer cannot force the US Treasury to take it back. They must find a willing buyer.
The current US government debt is $10-12 trillion. The government can't be forced to buy back the bonds; however, if there is a significant amount of bonds for sale it will make it difficult for the government to issue additional debt.
Yes, the seller would traditionally receive USD when they sell US Treasuries.
No, not to redeem it. But it can be sold.