"In my opinion, Atman/Brahman is some inaccurate Vedas' rip off theory from Theravada's distant cousin, Mahayana and its doctrine."
No, it didn't. It is quoted in the Pali Canon as the view of some sects (probably pre-Upanishadic since it isn't expressed in the Atman/Brahman terminology):
""He assumes about the view-position — 'This cosmos is the self. 8 After death this I will be constant, permanent, eternal, not subject to change. I will stay just like that for an eternity': 'This is me, this is my self, this is what I am.'"http://www.accesstoinsight.org/tipitaka ... .than.html
"In the abhidharma, our mental consciousness (ignoring the first five), after analysis and dividing, are nothing more than seventeenth moments of thoughts which are also subject to arising and cessation. In the other words, they are conditioned and cannot be a permanent unchanging self like Atman is defined as."
Traditional Theravadins are going to hate me for this, but I think they would be better off by jettisoning the Abhidhamma as it is rather illogical and leaves them susceptible to attack from other traditions (You can pretty much destroy the Abhidhamma when armed with Kant's Transcendental Unity of Apperception. Modern scholars have also destroyed the notion that it came from the Buddha).
As I indicated in the previous posts, Ajahn Thanissaro's approach totally destroys the Upanishadic positions. It's too bad that traditional Theravadins hate the guy.