Surely you don't think that he remained in that little room until 2 minutes before the (late) motorcade came by then went up to take a shot?
You're asking to defend some loon's idea that Oswald was in the lunchroom minutes before the assassination, when no LNer even promoted anything so ludicrous? And somehow it now takes Oswald two minutes to get through the lunchroom door (he can't leave the lunchroom closer to the alleged 12:25 time when Norman/Jarman went into the building?).
Why would Oswald care if anyone was on the stairs or even on the sixth floor when he went there? If they weren't in the Sniper's Nest, then no problem. He left his prints all over the SN and on the rifle he owned and left behind. He was no doubt surprised he got out of the building alive. To celebrate his lease on life, Ol' Os' done went and murdered a policeman
Those are the actions of a lone killer who's remaining options (as he saw it) were death by policeman, being murdered by another prisoner while awaiting trial, suicide or state execution.