I believe SA Lems Johns did Not get back inside the LBJ SS Follow-Up Car. Johns eventually arrived at AF1 AFTER LBJ had been there awhile.. If Johns had jumped back inside that Follow-Up Car he would have arrived at AF1 when LBJ did. (Johns was assigned to the LBJ SS Detail). I think Johns arrived at AF1 when Jackie did. To me, there is No Way SA Lem Johns could have jumped out of that vehicle in the middle of Elm St and run up the street toward the LBJ convertible and Not be captured on Any assassination images or be sighted by Any of the eyewitnesses inside Dealey Plaza. That's just flat-out Unbelievable. Wiegman filmed JFK all the time so he would know the different SS Agents. I believe Wiegman's story of seeing SA Lem Johns at the Top of the Knoll.
That's why Trask's wording regarding the Sitzman Interview is Important with regard to the Factual Record. Trask very well could have used his own connect-the-dots thought process. He also did this at times in "POTP". Like I said before, it is Not unusual for assassination film snippets filmed by different camera men to be Spliced together for JFK Specials or VHS/DVD presentations.
We don't even know if Lems got fully out of his car, let alone halfway to LBJ, so I am assuming he did not(especially the latter) because of exactly what you said, there's no evidence for it.
Wiegman met Johns at the TM, ask yourself how and why? Why did Johns group go to the mart and not follow the limo to PH? Seems obvious to me, his driver lost the lead cars and so went to the designated next stop like every other vehicle behind them.
Wiegman then tells of Johns himself going around gathering intel and when he hears the news, tells Dave, of all people and invites him to join him, Askins as well, on a trip to the hospital in a police car(which I thoroughly believe btw). Nice to have friends.