Been thinking about this as it is important to establish the order people used the elevators. Apart from the assassin(s) we know four TSBD employees are up on the 5th and 6th floors at the time of the assassination. Jarman and Norman arrive together and must be considered a single entity for these purposes. It seems clear from the testimonies Williams is up there first - he washes up, gets his lunch from the Domino room, heads back up stopping to get pop from 2nd floor lunchroom. He uses the east elevator.
I would place Dougherty next - he goes down, eats his lunch in the Domino room then 'returns to work' after "just a short length of time". He uses the west elevator.
Jarman and Norman don't go up to the 5th floor until shortly before the motorcade arrives in Dealey Plaza. Hank Norman notes that they were outside until they heard news that the motorcade was on Main St., some time after 12:23 (the only reference I can find to this is a second-hand one from "Reclaiming History" pg34-37). This ties in with Jarman's recollection of standing outside "until about 12:20, between 12:20 and 12:25." This has Jarman and Norman arriving at the elevators about "12:25 or 12:28" as he states in his WC testimony. The big problem with this scenario is that both elevators should be up on 6 but they're not. Jarman is adamant the east elevator is up on 6 giving the strong impression the west elevator is on the 1st floor when he and Norman get on it. There is no mention of calling the west elevator down.
Need to figure out who uses the west elevator before Jarman and Norman get there. Obviously someone coming down to the 1st floor from a higher floor.
Some points to consider. If Dougherty goes back to work, getting stock from the upper floors, using the west elevator He does so after Jarman and Norman ascend. I agree with your timeline regarding the movements of Jarman and Norman. Truly mentions seeing them depart the front of the TSBD with Givens, crossing Houston then returning to go to the back of the building. Seems they changed their minds leaving Givens to continue alone to the parking lot.
The Dr Pepper machine was on the first floor near the back stairs. If the bottle was his he did not need to visit the second floor lunchroom.
The first time Williams mentioned his trip to the 6th floor was to the FBI on 11/23. On that occasion he said he used the back stairs to go down to join Jarman and Norman. His 'story" would evolve significantly over the succeeding months.
I recollect a statement by Frazier that he and Jack went to the front of the building late to watch the parade. Jack gave his reasons for changing his mind and going to work instead.