They would have passed the domino room about 12.22:30 and arrive at the SE corner of the fifth floor about 12.25pm.
That could very well be correct. And I think we can all agree that even if Oswald saw them walking towards the elevator at that time (as I think he did), it, by itself, would not provide him an alibi for the shooting at 12:30, as he could have made it to the 6th floor within the time frame.
If only there were no witnesses who saw activity in the SN window as early as approx. 12:15.
If Oswald does not begin his trek from Domino room until seeing Norman/Jarman get on the ground floor rear elevator at 12:23, then Oswald has to cross about 100 ft of 1st floor, then go UP 5 flights of 18 step staircases, and across 5 landings of about 15ft length on the diagonal, that would probably average out to at least 20 sec per floor (due to fatigue climbing against gravity) thus Oswald reaching 5th floor landing approx 100 sec or 1 min 40 sec after 12:23 which = 12:24:40. So yes, it is possible for Williams and Oswald to have missed seeing each other on the 5th floor landing ... HOWEVER..
Oswald still has ONE MORE flight of stairs to ascend, and 180 MORE ft of 6th floor to trek across and also must get his rifle from some hiding place, and that would therefore make it highly improbable to have reached the SN window by 12:25, let alone have time to select which boxes he wanted to use, and move/slide 2 of the boxes up against low wall, and lift a 3rd box onto the window ledge, and then duck out of LOS just before Bronson film starts approx 12:25.
Unless someone can show Bronson film did not start until 12:26