You go on to claim that ""This definition exactly describes atheism"
I claim it does not.
Most of the atheists I know barely think or talk about it. Thinking six friends who I know are atheists their "interest followed with great devotion" are music, martial arts, their dog, civil war reenactment and beer. In fact they just describe their lack of belief as "I don't believe in God" and/or "I'm not religious"
Your argument is akin to saying "failure to participate in bowling, is a form of bowling."
Your second alternative definition is much better: “a system of belief establishing worldview, and the behavior patterns derived from such belief.”
But again, this is not the core of religion, it's sort of using the word to describe an effect of religion. Using this definition being a "Conservative" "Libertarian" or "Socialist" would be a religion. But, again that's an unconventional use of the word.
Can we really generalize about the "behavior patterns derived" from Christianity? With almost 2 million Christians, I think it's hard to generalize like that.
I think a lot of people's behavior is influenced by religion, and for some it's the overwhelmingly most important influence. But for a lot of people it's not the most important and things like culture, politics, social networks, and education have more influence on their behavior than the religion they profess.
Strength of belief is essential here.
The atheists you describe are not truly atheists, they are agnostics. They don’t know if God exists and they don’t care. If you don’t see this now, you will see it if you engage them, or have a Christian apologist engage them, in a friendly but serious debate about the existence of God. The vast majority of self professed “atheists” also don’t know what atheism truly is, and will admit they are agnostics after being subject to a real debate.
Atheists feel very strongly about their belief system, and will drive their arguments to the end of the earth. This belief system is a religion in that it purports to answer questions of cosmology, and it outweighs all other beliefs in magnitude of intensity.
Now for the link found in neurophysiology and neuroanatomy. Atheist Sam Harris found in a study a few years back, using functional MRIs, that the part of the atheist brain activated during contemplation of their belief is the same part activated in the Christian brain when Christians contemplate theirs. Contemplation of beliefs included reading respective statements, by atheists and Christians respectively, such as, “The God of the Bible is not real” and “The God of the Bible is real.” The verbiage of these quotes might be off by a word or two, I’m going by memory here.