I agree with most of your reasoning and conclusions (especially that human and animal moral worth exist on a continuum), with two exceptions:
– I believe there is a truth to the matter of moral worth. I believe ethics stems from the existence of conscious, sentient beings, and that an individual's moral worth is proportional to some degree of sentience, which can be well defined by science, but hasn't been so far.
– I also believe that there are circumstances where farming and slaughtering animals may not be a net moral evil, as long as living conditions and slaughtering processes are humane. This has a lot to do with human exceptionalism, not so much in moral worth as in our knowledge of our own death, our ability to understand potentialities and risk and to dream and plan the future and our gigantic social circles, none of which animals have to the same extent.

For more on these ideas, and on how to understand ethics scientifically, please take a look at my Steemit blog as I think you might be interested. Only one post so far but lots more to come :)