We managed to get it all done! I filled up the can with lots more cuttings from our giant overgrown rose bush (tree) and a branch from the yucca tree next to it. I just finished painting the first coat on the attachment (John started, but I had to intervene because I'm anal about the brush strokes), I taped the hallway this afternoon, and John helped me hang the mirror just before bedtime. Phew!
I think the mirror makes this corner look much more complete, but I still feel like it's missing something. We are both leaning toward painting the mirror, a la this look (but not this color; maybe coral):