Thanks for your interesting comments/questions, Marchesk. Let's see if we can discuss some of the points you've raised.
"Plants don't count but horseshoe crabs do because ...?"
-I think we can actually count plants too, if you like, and this shouldn't adversely affect the argument.
"Wouldn't the statistics be heavily in favor of being some bacterium or virus over any other life form?"
-If we follow the same logic that leads to the conclusion that animal minds don't exist, then yes. The question would be: can we interpret these statistics to mean that bacterium, viruses, etc. do not actually exist, such as if the external world were an illusion?
"Afterall, what makes the human mind more unique than that of an octopus or a bat? Just because we value abstract reasoning more than being able to see with sonar or camouflage into the background?"
-This is a very good point, one which I probably should have addressed preemptively. I think there are many things that make human beings more unique than any other type of life. When I say this, I don't mean that it's intrinsically better to be a human, or that humans are objectively superior to other life. All I mean to say is that we are set apart by our reasoning abilities in a way in which no other life form is distinguished by any of its unique attributes. Consider the fact that there are only human-run animal zoos, and there isn't a single human zoo operated by animals. Of all conscious creatures, I would say that humans are the most dominant. This, I would argue, is what sets us apart. If an extraterrestrial were to visit earth, I think he'd conclude that human beings are relatively "in charge." When he would return to his home planet, I think he'd first tell his fellow aliens about human civilization, and then later he'd discuss bats, whales, giraffes, poison ivy, etc. Now, you can argue that I'm looking at this from an anthropocentric perspective, and that nature disagrees with me. You might even be right, so let me change my initial question a bit. Since you aren't convinced that the human mind is more unique than that of other creatures, let's pretend that humans were more clearly different from other life. Suppose you lived in a world where there were only ten human beings including yourself, but a trillion identical creatures of another type. What would persuade you that these other creatures actually exist? Since every living creature is identical to each other, except for you and the nine other humans, we could definitely say that there is something exceptional about humans given the sheer number of the other living type of creature. Would you conclude that these other creatures don't exist, since it's enormously unlikely that you would've been a human if it were possible for you to be one of them? If not, why not?
"It's incredibly unlikely to be able to use your mind to change the color of every single skin cell, therefore, only cephalopod minds exist."
-"I think, therefore I am." I don't need to worry about whether or not only cephalopods exist, since I know that I exist. At least one human, me, must be real. However, from the perspective of the cephalopod (if these creatures were capable of philosophical thought, which they aren't), this argument would resemble the logical behind the argument I introduced in my initial question. One concern is that the cephalopod would need to assume that the ability to change the color of his skin sets him apart as a winner of a lottery among all life forms. Is there any reason for the cephalopod to be surprised by the fact that he is capable of a behavior that isn't exhibited by the vast majority of other creatures? I would say not, since countless other types of organisms feature equally remarkable yet unique abilities. Let's say, just for the sake of argument, that we grant that the cephalopod is truly the winner of this lottery, that he is actually somehow luckier or seemingly more important than human beings and all other life on earth. How do we convince him to believe that the external world is real, given that believing this would require that he believe that something extremely remarkable happened. If the cephalopod asks you if he is justified in thinking that the external world not existing is more plausible than his winning a lottery against all odds, what would you say to him to convince him otherwise?