The gross signs that doctors measure are just a pragmatic method that does the job the doctors are interested in (saving lives), not a definition of consciousness.

If you already acknowledge this then why did you bring medical tests up to begin with?

Everything we observe about how this experience is modulated by physical circumstances suggests that it is specifically a physical process of the brain

Some physical process in the brain may just as likely simply be involved in organizing and amplifying consciousness as be totally responsible for it.

we observe nothing to suggest that it is something a rock could have

What specifically do we observe in other people to suggest that they could have it? Isn't the entire point of the "p-zombie" concept to show that nothing we observe about other people could possibly evince consciousness?

"X implies Y, therefore Y implies X" does not work as an argument, especially when we already know Y ("humans are conscious") to be true

The whole process of model-building is to find X's which imply Y's where Y is already known. That's pretty much what science is, right? Nothing about the world is known purely through deduction or induction. It "does not work as an argument" insofar as it's not a species of deductive (or, narrowly speaking, inductive) activity; but that's not to say it's epistemically inert.

Any number of things imply Y, including, for example "only humans are conscious", "all terrestrial animals with a nervous system are conscious", or "any physically faithful simulation of a conscious entity is conscious." I don't see any reason to favour "everything is conscious" over any of these.

Because "everything is conscious" is vastly less arbitrary than any of the other choices you've identified.