I agree with much of what you said, except for the comment about the Viet Cong. The VC pretty much ceased to exist as any kind of effective fighting force in 1968, with the Tet Offensive. But you are right about the 1972 bombing: Had we kept it up the NVA would have thrown in the towel. Unfortunately, our strategy with the 1972 bombing was not to get North Vietnam to surrender, but to get it back to the Paris Peacetalks to agree to a cease fire (which was what happened).
There's no substitute for a crushing victory.