Studs never seem to be where you want them when putting up an oversize mirror or installing a wall-mount cabinet. And your usual hangers, like nail-in picture hooks or large-thread drywall anchors, tend to rip out under loads greater than 25 pounds. No worries. If the wall hanging is wide enough to span two studs, you can support it with a French cleat, even if it's off-center.
Safely work above stairs: Open a step ladder and place it at the top of the stairs. Then position an extension ladder on the stairs, leaning it against the opposing wall (pad the arms to protect wall paint). Construct a platform to span the distance between the ladders by screwing 2x4s to a plywood deck. Nail stops to the ends of the 2x4s to prevent the assembly from sliding off the ladder rungs. thisoldhouse.com