The zero article is used for proper names. No article is used for adjectives. These two rules are all you need to understand the examples given.
It was hard to recognize this pile of tissue as human.
If we look up "human" in the dictionary, we can see an entry for the word as a noun and as an adjective. If we used an article here, "a human", that would tell us that we are using the noun version of the word. Because we are not using an article, that tells us we are using the adjective form of the word.
Compare to the following sentence, and you'll see the same structure:
It was hard to see the man as ugly.
Your next question:
Here, there is no adjective form of the phrase "Anatomy lab". Thus, per the rules above, if there's no adjective it must be a proper noun, otherwise known as a "name". What is the meaning of this as a name, instead of a simple "adjective + noun"? It refers to a school subject. Compare to the following sentences:
Math becomes less an exercise in frustration and more a pleasant pastime.
Gym becomes less a torture pit and more a refreshing activity.
Social Studies becomes less a time to talk about the world, and more like nap time.
When talking about a school subject in this way, as a name, it refers to the entire experience of taking that class, but not any specific day or moment. It becomes more general.
And your final question:
that interferes with happy hour.
This extends the previous answer. Once more, "happy hour" in the dictionary is only a noun, never an adjective, so the only answer can be that we are using it as a proper noun, just like with school subjects above. And the usage is just like school subjects, too, making this a wonderful parallel. "Happy hour", with no article, refers to the entire concept of the time in a bar where prices are reduced, not just a specific moment on a specific day.
I hope this helps!