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.
Perhaps I read. in another book that LBJ thought that Mc Cormack was planning to be sworn in, and that had him soiling his skivvies..... But I thought it was in
The Death Of A President....
I'll have to admit that it was Manchester's book that opened the door to the case for me....I hadn't read the book which had only been published about a year earlier, when my brother and I got into a discussion about the coup d' etat. I knew very little about the murder but I felt that we had been handed a dog turd ( the WR) presented as a candy bar, by LBJ's "Special Blue Ribbon Committee" but my brother told me to read Manchester's TDOAP And so I did..... And that opened the door. I wasn't at all convinced by Manchester's book but I'll have to admit the man presented many facts .......