And if they don't send? With electronically stored fiat, the feds can just ask the bank to give them the money right out of their account, and I'm sure the banks would cooperate. With paper cash they can just hold their arm while the SWAT team go on a rampage through their bedsheets for the cash.

What does one do for Bitcoin, short of a hard fork?

That would require a hard fork or the key to the sending utxo.