About the latent print (my opinion);
As an obvious first step in this investigation, after the stage where the bodies were finally removed, the investigators brought in a luminol team, among many other technicians.
For there to be one invisible bloody footprint, there had to have been others. So, I assume that at least one foot of the murderer stepped in blood and they already had a luminol trail, so that from whatever point the killer stepped in blood, they could see it (keep in mind that luminol turns up nearly invisible footprints - the first couple of prints are likely visible).
Each time the murderer takes a step, there's less blood on the bottom of his shoes, as that's the basic mechanics of a liquid on the bottom of a shoe. As many of us already know, a UV light is used, along with luminol to produce images like this:
(That's not from this investigation). So, after interviewing DM, and probably after much of the rest of the investigation was completed at the house, they brought in an Amido black technician.
In the following image, the blue footprint was invisible before Amido black was used:
dps.mn.gov
So, my view is that LE had a trail of footprints that gradually disappeared. Luminol picked up some of them, which we'll see in creepy and terrifying succession at the Prelim (most likely). There will likely also be actual bloody footprints at the beginning of the sequence, which will tell a story. First, there will be the bloody prints, then the luminol footprints (not visible to the naked eye but visible with luminol) and then the Amido black enhanced footprints, for footprints with even less blood protein in them.
The enhancement made by Amido makes proteins (not just blood) more visible, but in this case it was blood protein that made the Amido black react.
And so they got a latent print from right in front of DM's door. I would assume that a footprint expert can speak to whether the foot paused or not, if there's enough outline to the print (it looked pretty good to me). But it doesn't really matter. They have physical evidence to back up DM's claim that someone walked past her room.
A good footprint analysis (even if the shoes are never found) will reveal (obviously) foot size, the turn of the foot while walking (an aspect of gait), the sex of the person (with above 95% certainty). If the same footprints are found somewhere outside (there's a photo on the MSM of the day that investigators were in the back yard, seemingly examining and marking a footprint as evidence, IMO) then the story gets more detail. It's also possible to say something about how quickly the person was moving by looking carefully at all the prints. It also gives a rough estimate of height. The value of this is only in excluding many people, rather than in narrowing down to one (unless the shoes are found).
IMO
I feel so badly for DM, as it must be terrifying to be such an important witness.