Lyndon Johnson and his people said his bugbear was the possibility that someone on the other side would go nuclear if a "tripwire" were passed.
I wonder if that is so, even if we did receive a back-channel communication from someone.
It is never far from my mind, that Lyndon Johnson was an enormous ego with an agenda, and that Vietnam was an inconvenience to his efforts to build a "legacy" worthy of his ego needs -- IMHO, I think he sought parity with Franklin Roosevelt (he was surpassingly vindictive, and always sought to "even the score" in some way with every mentor he ever had) and was hoping for a shot at Mt. Rushmore.
Johnson basically didn't want to fight a war in Vietnam, and he absolutely refused to compromise his domestic legacy-building agenda. His ego tied his hands more than the Russians did, IMHO.
LBJ did even more damage to this country than Clinton could even dream of, and that's saying something.