Can't unscrew the Penelope screw (at the bottom of the iPhone)

you can try a strong magnet (like the kind used in hard drives - at least 3.5" or bigger) to pull the screw out, but if there's even a small part of the screw flange in the threads, it's not going to work. something sticky like a rubber cement ball or even duct tape (if the screw is sticking out far enough). then there a vacuum, but you'll need to constrict the nozzle down to just larger than the screw head to focus the air flow and increase the suction - the downside to this is that you'll probably lose the screw if it does come out, but you're already short one so you'll probably need to shop for a replacement pair anyway (very easy to find on ebay).

other than that, i would not recommend simply using a spudger or pick to try and separate the display from the base because if the screw is still holding on to the bracket on the display, you'll likely break that off and now you'll be unable to fully secure the display side back without replacing the whole thing (you can't just replace the screw hold down bracket