How do I screw the key piece back in?

Hello, seems to me you'll have to remove the door panel. There is no other way I know of to fasten the handle parts. And, at that point - you might as well finish the repair! This is where a workshop manual matters, getting the instructions needed to do the job so you aren't stuck. Removing the door trim panel is very easy, as long as you have a trim tool to prevent damage to the trim and door and find all the screws and fasteners, removing them all. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops