I reckon that putting a corpse in a reaper costume might be a suitable way to transport remains in late October without drawing much attention. Halloween decoration or scarecrow
The costume may have been the UID's, but may also have been a convenient way to disguise remains for transport, particularly given the time of disposal in late October. I reckon it would be easy to pass off as a decoration or scarecrow to anyone that might catch sight (assuming they also don't catch scent).
People in both rural and urban areas of the US have routinely written off deceased persons as decorations or pranks.
As for the state of the remains, they must have something usable to have ruled out Alicia Navarro so quickly. I don't know that DNA would have been processed this quickly, so perhaps dentals are available or there is a specific physiological characteristic that made the rule-out possible.
There might not be a skull, or perhaps the skull is too damaged to facilitate a recon. You'd think that someone concerned with removing head/hands wouldn't leave any hair, even shorn locks, considering that we can now develop full profiles from rootless hair. But this is a quite new development (publicized as of 2019) and...well...I wouldn't expect too many murderers to be keen readers of science journalism. Considering the uniformity of the hair pieces (curled, roughly the same length, considerably darker than sandy), it might have come from a decomposing wig. Coloring could be bad photography/editing, grime, or blood.
Then again, thinking about the remains, there might not be all limbs/head present. I think it would be awfully hard to stuff a 5'1" body wrapped in a tarp into a 4' box. On the other hand, 5'1" is an incredibly precise height, even for skeletal remains, to not have all pieces present. Regardless of height, I think weight will reflect normal or low BMI if the cardigan is the UID's. This seems to the junior/regular model, rather than the plus-sized model of the Hot Topic cardigan.
Last, I've found that even UIDs with short PMIs (like hours or even 1-2 days) are often marked as "skeletal" and "decomposing beyond recognition". One of my biggest gripes about NamUs is that the system allows for "lazy" drop-down selections without mandatory explanatory notes, as well as for incomplete entries without spell checks (the frequent number zero subbed for the letter "o" is infuriating and prevents accurate searching).
Unfortunately, I think the UID may not have a correlated MP report until school resumes fully and administrators check attendees against 2020 notified moves/drop-outs. I think the young age suggests familial violence or an unreported foster home runaway. Undocumented or migrant labor is also a possibility. It's just so odd that the person who discarded her had hundreds of square miles of uninhabited desert, yet chose a place where she'd necessarily be found. Was fast disposal (or fast relocation) a necessity? Did anyone suddenly put property up for sale, or been evicted (legally or illegally) in late October within an hour radius?