According to the CTers who post here, Oswald wasn't on the 6th floor (or at least they suggest there is no evidence of such). And some even claim he had an alibi. So why would anyone need to make sure Oswald was in DP to frame him for the crime? They drop the same rifle with his prints on the grassy knoll, arrest Oswald somewhere else, and coerce any person who could give him an alibi into silence. All things CTers regularly suggest did happen to frame Oswald in the TSBD.
The conspirators just have to make sure he doesn't have an iron clad alibi, one that he can use to clear himself, one they can't manipulate/control or make disappear. Then they can put him anywhere in Dealey Plaza - the picket fence, the underpass, the sewer - and plant the evidence for his acts.
This idea that two months before the assassination they plant him in the TSBD and then two months later everything works out so they can frame him is an idea I find completely impossible. So many intermediate steps have to go right, so many uncontrollable things, that it's not even remotely possible. But this view is based on the idea that there were/are limits to what they can, how much they can control.
In conspiracy world anything is possible. As Hofstadter said, the conspiracy view "Believes it is up against an enemy who is as infallibly rational as he is totally evil, and it seeks to match his imputed total competence with its own, leaving nothing unexplained and comprehending all of reality in one overreaching consistent theory."
Again: "imputed total competence."