The Dems and media can try to shift the blame to Nixon, but facts are facts and Nixon took office years after McNamara did this.
Why was the North off limits?
Uh, I think we need to look back a little further, to the Eisenhower administration, for “who got us into Vietnam” (in the 1953-54 period in support of the French against the Viet Minh). Then installed Diem as Our Man in Saigon, had him create the faux independent nation of South Vietnam (RVN), so the legitimate government in Hanoi (who had represented Viet Nam in the Geneva Conference of 1954) could be considered to be “invading” a sovereign state. The ROE were consistent with a limited war, which it always was.
Nixon did not create US involvement in Indochina, nor did he create Project 100,000; it was well underway when he took office as POTUS. But, he did let it run its course until 1971 when manpower requirements for Viet Nam were being dialed down.