Oswald likely scouted and/or knew from his experience in working in the building the various locations that offered the best combintations of seclusion and shooting angle. If it had turned out that a group of folks were on the 6th floor, he probably would have just gone up to the 7th floor where there would have been no one. I think the ledge under the 7th floor window might have been the reason he didn't do that in the first place. Just speculation, though, as it turned out to be unnecessary.
There are a lot of "ifs" that determined the outcome. If Marina would have agreed to look at apartments that day for one.
I don’t think the 7th floor windows can be opened and I don’t know about the roof. Like you said, LHO probably had searched for an alternative. He obviously saw they were working on that floor.
LHO ended up choosing the best location with the way the road slopes away from the building. He would not have had to adjust his aim much to correct for the movement of the car. No other window in the building would have worked as well except maybe the floor below.