I am so sorry for poor Sam and for you - looks like it was a nasty fight he got into. While it could have been many of the predators you listed, I would lean towards the dog family culprits - mainly dogs and coyotes. Whatever it was grabbed him from behind and likely dragged him as you mentioned so it had to be a larger animal and the backside injury is classic for canine attacks on cats. I suppose a great horned owl is a possibility too though taking a full grown large cat is not common (though not unheard of). Can't completely rule out another cat, although cat fights tend to cause head, face and ear injuries (obvious scratches, maybe a bite on an ear, leg or foot, but rarely a tearing, gaping hole). And some of the other "predators" you listed really are not predators for cats - I have seen plenty of cats eating out of the same food dish as a possum and have seen cats in the yard with skunks and raccoons - possums, skunks and coons just don't seem to look at cats as prey although if one were sick (rabies), it might attack. Same goes for foxes too - cats and foxes tend to be similar enough in size that they are more competitors for food than predator/prey towards each other. We had a fox that took up with our barn cats during the fox's declining years and they all got along fine. Long story short, I would lean towards dog or coyote. Whatever it was, best wishes to Sam on a speedy recovery.