This doesn’t mean non-native predators pose a larger threat. If anything, it suggests the total number of opossums killed by native predators could be greater than the number killed by non-native predators, which would weaken the argument.

This is irrelevant. It doesn’t mean other non-native species pose an even larger threat to ringtail opossums than foxes—there’s no indication those other species even prey on foxes.

This suggests the opossums killed had diets that were typical of wild opossums, not that their cause of death was typical. It doesn’t disfavor the leading alternative hypothesis, a food scarcity, because it doesn’t imply the opossums killed were able to find food in the wild.

This is irrelevant. The conservationists explicitly blame non-native predators for the ringtail opossum’s endangerment, not species that compete with them for food.

This rules out an alternative explanation for the opossums’ deaths: that the opossums raised in captivity were killed by foxes in large numbers because they were unusually bad at protecting themselves.