The Oswald shooting has been duplicated by many so not impossible, difficult but not impossible. I tend to believe there was a second shooter however for a host of other reasons.
Rifle on the roof prior, extremely unlikely. It's a light-colored roof and any object would be easily seen from the windows in the building next door. The rifle can be easily seen on the roof in post shooting videos. It's black on a white roof. It does have a collapsible stock but that only shortens it by maybe five inches or so. There does appear to be a large backpack on the roof also however that is probably big enough to conceal a collapsed AR.
Interestingly, there is a door that opens onto the roof complex from the two-story section. That would explain how the person who was supposed to be on the roof got off so easily when he/she got too hot. Straight into the air conditioning.