Headboard fell off. Suggestions on how to re-attach?

I'd definitely try a hanger set up. I think you might have attachment issues with the mirror clips because they need to be joined accurately. If it was me, I'd visit a big box store and buy the Command hanger kits that come all in one blister package. The Command hangers stick very well to finished Luan, can't say how they do with wallpaper, maybe the package has information.

If not Command strips, it would be two picture hangers and two screws, job done.