LBJ wanted to take the oath of office in Dallas (even delaying their flight back to Washington DC for it)
Keep reading and you'll find that John Mc Cormac was hurrying back from lunch after learning of the death of JFK and Mc Cormac assumed that he had become the acting President and needed to return to the House to be sworn in. LBJ heard that Mc Cormac was about to be sworn in and he nearly soiled his skivvies.. THAT"S why he insisted that a Federal Judge be brought to AF1 immediately to swear him in.
I have already read this account and you (as usual) have it all wrong. I am assuming this is the passage you are referring to:
For Walton, Moynihan, Horsky, and Duke, mourning thus began early; for John W. McCormack the confirmation was a private anticlimax. The Speaker had still been in the House restaurant when two reporters came to his table and said that Kennedy had been shot. Other reporters and Congressmen then began to dart up with bits and pieces of information. The appearance of priests convinced McCormack that the President had succumbed. Then, in the next minute, he was told that the Vice President had been shot and, in the minute after that, that Secret Service agents were on their way to the Hill to protect him. Although the first report was inaccurate, the second was true; under the succession act of July 18, 1947, inspired by Harry Truman’s affection for Sam Rayburn, the Speaker (rather than the Secretary of State, as in the past) was second in line of succession, and if both Kennedy and Johnson had been murdered, Rayburn’s aged successor was now President of the United States. At 2:18 P.M. in Washington the possibility seemed very real. It struck McCormack, he later recalled, with “a terrific impact.” He rose unsteadily from his chair and immediately suffered a severe attack of vertigo. Linen, waiters, tableware swam before his eyes; he thought he was going to lose consciousness and tumble to the floor. Passing a palsied hand over his eyes, he sank back to his seat, and he was still there, trembling, when a Congressman called over that Johnson was unharmed.McCormack had been told that LBJ had also been shot (see the part that I underlined). So your idea has no merit.