Mining of Haiphong harbor (which Nixon tried later) would have been about as succesful as the British closure of Boston Port in the 1770s. The problems in SV were internal in SV as nearly every ARVN officer was involve in coup plotting one time or another rather than fighting the commies.
An invasion of NV, of course, would have been a disaster and brought massive Chinese intervention (a la Korea).
I don't recall recommending an invasion. Of course that's what the democrats would have done. My point is we fought in the country we were trying to defend. the country where the war is fought is the one that get destroyed. As for China, it depends on whether you are credible in your intention to win.