This doesn’t help us. The premises have already told us that the skeleton shares the T. rex's features, meaning its head is \"oversized\", regardless of the specific ratio. So knowing the specific ratio wouldn't help us determine whether such features emerged as adapations to size or not.

This would be useful in evaluating the argument. If the skeleton is a dinosaur who died as a juvenile, then perhaps that dinosaur was simply a young precursor to T. rex who would eventually have grown to a great size. This would undermine the author’s claim, supported by this skeleton, that certain T. rex features weren’t size adaptations.

We don’t care if T. rex was the single largest and heaviest prehistoric predator. We just want to know whether its size and weight caused it to develop particular features or not. Whether or not there were larger predators, possibly with different adaptations, is irrelevant.

This doesn't help us evaluate the argument. Remember that this dinosaur skeleton is earlier than T. rex. Regardless of whether it was a forerunner related to T. Rex or not, we would still have to explain why the characteristic T. Rex features developed for a dinosaur that was nowhere near the T. Rex's size and weight.

Irrelevant. We don't know how large the species T. Rex preyed on were: for all we know, perhaps it tended to prey on smaller species. So knowing whether or not the smaller dinosaur with T. Rex-like features preyed on the \"same size\" of species as the T. Rex doesn't actually tell us very much. It certainly doesn't help us determine why the T. Rex's features arose, and whether or not they developed to accommodate its size and weight.