Kennedy did at 1:00 pm on November 22. However, LBJ was not sworn in until 2:36 pm. Was LBJ the acting president on the 22nd, or did presidential succession mean that the speaker of the house was president?
No, power never would have devolved to McCormack. LBJ was President the moment JFK died under Article II of the Constitution:
In Case of the Removal of the President from Office, or of his Death, Resignation, or Inability to discharge the Powers and Duties of the said Office, the Same shall devolve on the Vice President
Under the constitution, it's very clear that the VP takes over if the President dies, so Presidential succession would not be an issue as long as the VP was alive and not incapacitated. He might not have taken the Oath of Office yet, but there wasn't any question that he would - so there was no need to look further down the line of succession.