The difficulty reaches a maximum when the target hash is 0. If SHA256 is broken so badly that you can calculate a set of inputs to produce a particular output (

The difficulty reaches a maximum when the target hash is 0. If SHA256 is broken so badly that you can calculate a set of inputs to produce a particular output (0 in this case) then bitcoin can't make it any harder. You could mine a block as quickly as you could calculate valid inputs.