Your premises are spurious. As for the first, cannibalism is not that uncommon among mammals. Chimpanzees, our closest relatives, have been known to practise it. And it was a traditional practice among numerous indigenous populations, including those of New Zealand and Papua New Guinea.
As for the second:
"Humans are smarter than other animals. Yes, we've seen startling examples of animal intelligence in unexpected species (the piano-playing octopus), but they aren't designing the Sydney Harbour Bridge. When we choose to take responsibility for caring for a non-human animal, we make decisions within the law for that animal, just as we would for a small child."
Completely disagree. "Intelligence" is subjective. Judging the "intelligence" of other animals by skills and faculties that humans value and compete in, such as piano-playing and bridge-building, but which would have no use to other animals, is setting them up to fail and viewing them through a narrow, anthropocentric worldview. It's a completely rigged game. But, of course, those in power are always the judges of what is "valuable", and of which qualities make some other group "worthy" - or not - of equal consideration.
In an athletic competition between men and women of similar athleticism, women will always fail, because men and women are different, and have different abilities. Many men have used this difference, among others, as justification for their treatment and dominance and "custodianship" of women. Another key long-supposed difference, highly relevant to this discussion, has been "intelligence". And, indeed, according to your examples, women come up short. Far less acclaimed/exceptional piano-playing in women's history, and certainly far less bridge-designing!
Henry Higgins sang, "Why can't a woman be more like a man?" (He also included the age-old tropes about female intelligence.) And all you're really saying is that what is "good" and "valuable" is what humans can do, and that non-humans inevitably can't be or act like a human and are therefore not as good/valuable/"intelligent". It's a circular argument, resting on a presumption of human exceptionalism. In a word, it's speciesism.
Maybe you can play a piano. But can you thread a spider's web? Can you tell the difference between the sounds made in the grass by mice and rats from hundreds of metres away? Can you communicate in bird "song"? Can you paint with all the colours of the wind?
I'll give you your bridges, but non-human animals build dams, hives, ant hills, spider webs, and plenty of other structures of complexity, functionality and "beauty" (again, subjective). And their designs have often been mimicked and incorporated by humans.
The best general working definition of "intelligence" is the ability to perceive and understand and adapt to environments. By that definition, every form of thriving animal life is equally intelligent. In fact, you could argue that humans' "ability" to not only adapt to and shape their environment, but to also go a long way towards destroying that very environment, makes us singularly unintelligent as an animal life form. Have any other animals made environmental conditions so inhospitable for themselves through their own activity?
And I couldn't care less what the law says, per se. Not about non-human animals, and not about anything. Lest we forget on this women's rights board that the law currently holds that a person may "legally" (and, in its view, "morally") change their sex via a Gender Recognition Certificate. Bah! The law is a human creation, designed to promote the interests of the applicable human society, and especially its ruling class. And human society is built on and takes for granted the use and exploitation of non-human animals. We may as well consult human law on the laws of physics and the origin of the universe, for all it has to reveal about the essential nature of non-human animals!