The Fed has only as much power as the world economy allows it to have. If they go crazy, the rest of the world may decide to drop the $ as a reserve currency and OPEC would drop the petro $. It would be disastrous for the $ and the US economy - but then and only then would the Fed control the $.
You really need to understand why the foreign sector has so many US dollars in the first place. Once you realize why, you'll understand that the largest holders of US bonds couldn't stop buying US bonds even if they wanted to (assuming they want to sell goods to the US).
Think about it, tensions between the US and China are higher than they've been in my lifetime, if it was really that easy to "drop the US dollar", wouldn't China already have done it? Or at least threaten it?
They haven't because that's not how it works. The RMB accounts for about 2.76% of global reserves. The reason that Saudi Arabi or OPEC as a whole cannot adopt the RMB or any other currency is there simply isn't enough circulating in the global economy to do all the business that wants to be done. Sure Russia and and a few other BRICS nations have flirted with using the RMB, but that market is miniscule compared to the US dollar and it cannot grow to compete with the Pound or the Euro never mind the dollar.
Now why are there so many US dollars flowing around the world? Because the US runs a MASSIVE trade defect. We are a large net importer of goods and a large net exporter of our dollar. not just for oil, for everything. If China wants to replace the dollar as the worlds premier reserve currency it needs to run a huge trade deficit, not to mention un-peg it's currency and become more transparent and earn the trust of the nations it wants to use it's currency globally.
Now honestly, perhaps one day that will happen, but not, as things are going, in my lifetime. Things are going ok for China now, but they are on the verge of a demographic crash and that's going to make things harder for China for the foreseeable future.