I'm a little confused about that; Albus claims the symptoms of Red Blight were what Merida had originally, but the image we see of Merida being laid to rest doesn't look at all like she's turned red or cracked open and become a hollow shell.
I think you're misinterpreting the explanation of the disease here. The person themselves isn't literally cracked open or hollow, what the doctor-hero is saying is that the splotchy redness, once it's taken over the whole body + killed the person, cracks off and fades into the air, leaving the original body. So he's saying that the red infection is like a shell, which when cracked open, reveals the body inside. That's what's being shown on page 17, with the guy with the cracked plates over his body, it all cracking away and just his (intact) body is being left behind.
I think the speech bubble that says "...a lifeless body, like a shell, as if the wax that once covered it peeled away" is probably just worded a bit oddly (potentially a mis-TL), making it sound like the body itself is a hollow shell, instead of the red blotches being the shell, which peel away and reveal the body. So Merida would've looked awful before her death, but after her death she'd be as pristine as she was shown here.