She's finished! My first proper bit of Teens-era undies!
I purchased some more muslin this morning, and immediately set to fixing the few things that were wrong. I lengthened the bottom skirt/ruffle by six more inches, hiding the join with the inset lace.
I finished the shoulder strap, unfortunately, with a different lace than the rest of the petticoat. The lace I had used was out of stock, and was unlikely to be carried again, so I made do with something that was fairly close.
It didn't turn out exactly like my inspiration piece - my lace is set farther apart, the neckline is a bit different, the skirt is shorter, etc, but I'm still happy with how it ended up. Next up are the chemise, which I already have cut out, and the corset, which I purchased a lovely white-on-white brocade for this morning. Hopefully they'll come together as quickly as this piece did! ^^