So you entertain the possibility that someone went to considerable trouble and risk to plant Oswald's rifle on the 6th floor to frame him for the assassination but then allowed him to be somewhere else at the moment of the assassination? What if Oswald had done what most others did that day and stood out on the street in the presence of his coworkers? Or had been in the presence of someone in the building? What a lucky break that Old Ozzie had no apparent interest in the motorcade and was hanging about in the empty lunchroom all by his lonesome. There is no way in this scenario that the conspirators leave Oswald's presence at the moment of the assassination unaccounted for.
So the idea is they planted his rifle and discovered it
before his co-workers were interviewed about him, about where he was and they were at the time of the shooting. If they give him an alibi - "Lee was with us on the steps at the time" - then their whole plan collapses. Or if he shows up in the Altgens photo or some other film/photo then it also falls apart. This is the murder of the president not the framing of a guy for murdering a gas station attendant. Everyone will be interviewed again and again and again. As they were. How do you keep it quiet?
His alibi would be discovered. They could not squash it forever. Or even quash it too.
Moreover, they then went through a similar process in framing him (for who knows what reason) the shooting of Tippit. Oswald is setup before all of the witnesses are interviewed. If they go to the lineups or are shown photos of him and say, "Nope, that's not the guy" or they or the media find eyewitnesses elsewhere who say "Oswald was with us" the framing of him here also collapses. Again, this is about the murder of JFK and not some 7/11 clerk. It's going to be gone over again and again.
This is just one part of this alleged conspiracy. They cannot do this. It cannot be done. Not and kept quiet.
But in conspiracy world - the world of "the CIA" and "the war state" and "the military industrial complex" - anything can be done. Their view of American institutions is so dark that they believe they could do this. And that all sorts of ordinary Americans would go along. It's a view of the US and of how real human being behave that we simply cannot reason with.
One of two things is happening here: either we lone assassin believers are complete fools, misled by "the government", conned into believing things that didn't happened, essentially living in a "Matrix" world; or the conspiracy believers live in a unreal world, a make believe world, the world of Garrison and Oliver Stone where nearly all powerful evil people and institutions - "the CIA!" "the military industrial complex! "Operation Northwoods! - covertly run the country. There is no middle ground.