What prevents the Fed from operating as many nodes as they need to overrule the "honest" node operators?
Bitcoin is the ultimate fiat in that it's not backed by anything tangible. The bitcoiners will say that the intrinsic value of bitcoin lies in the value the network provides, eg. facilitating uncensorable, permissionless transfers across borders. It is definitely not a given that a new coin without this utility will be a success. I'd bet against it given the history of altcoins that have attempted to do it.
This is completely compatible with the bitcoin network, any miner can do that today without forking the chain. However, if the Fed had say 85% of all hash power it would be extremely expensive for any other miner to mine a block, so the Fed would gain complete control over the network over time.