Sorry, as well, for your losses.
I went back to your post of still captures from the game cam. (looks to be about as vermin heavy as this location). I just wanted to respond, specifically, to your question:
Is it possible that a fox, or family of foxes, were responsible for the massacre, instead of dogs? It was my understanding that a fox carries its prey away to kill and consume. That might explain the 15 that were missing altogether, but the 7 who were just lying around, unmarked and dead?
Yes, it is. We witnessed the aftermath of a duck massacre at our neighbor's pond (winter/frozen over). Some of the ducks were `opened up', some were still alive (barely). The majority were dead but `unmarked' (bite marks on neck breast for most part). This work was done by a pair of foxes. The neighbor shot one and it had been carrying a duck head. This was the same neighbor who lost all thirteen of her Silver Laced Wyandottes last summer, at about four in the afternoon (first day for the pullets to explore the yard and she went inside to get them treats, she came back out and found nothing but a few feathers). This is the more common fox action (also a pair in this case). They kill, or kill while carrying off prey. They cache the prey at a safe location nearby and eat the meal there (I found both cache sites - nothing left but the poor girl's feathers - seven piles in one spot, six in another).
It appears, from your cam shots that you are in a wooded location. If the attack was made by foxes then, it is possible, by searching the woods nearby, you might find the cache site (might even find some buried) where your poor missing guys ended up, it would also give you all a starting point to hunting down the den (look on all areas with good southern exposure (like the southern downslopes).
Foxes are trouble, as they will attack at any time of day (particularly active a couple hours before sundown) and are pretty brazen. We had one of our GSL pullets grabbed up by a fox no more than twenty yards from where we were standing. We got lucky and the fox dropped her, some members here haven't been as lucky. It could be the foxes have been lurking and sizing your flock up while you all have been out enjoying them. I leave `noses' of weeds and bushes uncut when I brush cut along the woodline. When the chooks give the ground pred alert these are the first areas we scan. The foxes think they have cover, they are wrong.
In addition to predator proof coop/shed and six foot, welded wire fencing, we keep three live traps set at all times and at least two leghold sets out (foxes/coyotes). I have found, at this location, with our predator load (avg. 30 raccons a year - 18 foxes last summer/fall) that preemptive retirement of vermin from the target pool significantly decreases the overall frequency of predation.
Another very useful adjunct (if you are home and awake) is a baby monitor in the coop. We keep a cheap $20 monitor cranked up at night and when the chooks/turks so much as start to growl and whine we're able to spotlight and harvest raccoons/`possum before they even get to the traps.
Hang in there and take care.
