According to The Guardian, the DNA found in the trunk is not blood, which, of course, would greatly reduce its relevance to a homicide case.
I've posted previously that it's bodily fluids, as stated, not blood. Bodily fluids in a suffocation case would mean saliva, not negating the relevance to a homicide case. Not saying that this is a suffocation, just pointing out that the absence of blood does not necessarily greatly reduce the relevance.
And as reported, the charges considered are those relating to accidental death and then some kind of cover up--which is not the same as homicide/murder.
Finally, any item that was placed within a trunk that contained significant DNA from bodily fluids would have to be one of the following:
Clothing on which Madeleine had urinated, perspired, bled, vomited, or drooled/spit upon, and which had subsequently not been washed--and then was placed loosely and not within another container within the trunk, several weeks AFTER her parents had moved to different lodgings. (Or, in a more sinister scenario, the same, but placed within the trunk before her parents officially rented the car, with the DNA then coming from either Madeleine's actual body or from items near her at the actual time of disappearance.)
Blankets or beddings, same, which were not washed, placed within the trunk of a car rented after her absence by her parents. If the items, and this goes for the clothing, were placed there before she disappeared, then the parents would have been spending much more time with the othe parents present from Britain (the Tapas 9) then previously reported. This is not likely considering the fact that Madeleine and her siblings spent most of their days while at the resort, in the daycare/creche. The only time they seem to have interacted with the other adults is the unsubstantiated report of the early dinner on the day she disappeared, with Russel O'Brien present.
Finally: Item such as stroller or other equipment in which significant amounts of Madeleine's bodily fluids were also present, and then transferred. Strollers are usually collapsed somewhat to place in a trunk, so this would mean handles, wheels, sides, back of seat, etc. Again, somewhat unlikely for normal use. It would be much more likely that parental DNA would be transferred from the handles, but not the child's. As well, it is unlikely that several weeks after the child went missing, a stroller or other similiar item was put in the trunk.
With two year old twins and a four year old, the only stroller likely to have been taken on holiday would be a double stroller--and why take that, if the twins were walking and in the creche most of the day? Extra luggage, extra cost, extra hassle. I've flown to Europe with a toddler and a lightweight stroller, and I was definitely not the average traveler in terms of take along equipment. If we don't have pictures of the McCanns with stroller/push chairs, then I think we can conclude they did not pack them as well.
Again, my point is, it's far more work for the PJ to frame the McCanns, than it would be to just let them go. The conspiracy theory is the weakest one.