I can see how we were probably counter-productive there. So here goes:
Yes, Miserix was the original leader of the Brotherhood. While the era wasn't necessarily one of peace, the Brotherhood was a benevolent force that acted in the interests of the Matoran Universe at large.
The Makuta were always ambitious and deceptive, so it was not such a stark transformation (So why did they serve Mata Nui? A Number of reasons, ranging from a fear of Miserix/stepping out of line to the sense of duty that had not yet entirely eroded.). You're right, though. The Barraki were the first to attempt to overthrow Mata Nui, and it was their attempt that inspired Teridax to do something similar.
Yes. Earlier in the year, (79,100 years before Present Day on Spherus Magna), Miserix had sent Mutran in search of "Tren Krom". Tren Krom was essentaily Mata Nui's precursor, and he watched over the Matoran Universe until Mata Nui was created. After this, the Great Beings fused him with an island, permanently imprisoning him. Mutran was sent to find him to learn of his intentions; to find out if he was a threat. Instead, Mutran learned the true nature of the Mata Nui Robot from him. Mutran was the first to find out that they all lived inside Mata Nui. He relayed this information to Teridax, who used it to formulate his plan.
Later That year, Makuta Proposed his plan at "the convocation", an event wherein all Makuta were called to Destral in a meeting.
Yeah. Miserix stays true to his original purpose, and argues that the Makuta should suppress their ambitious, darker side. Teridax voices the opinion of the silent majority, though, and so Miserix is deposed, and Teridax becomes the new leader. This all occurs in the Convocation, just after Teridax proposed his plan.
Right again! Teridax's original plan was not to usurp Mata Nui's body, but to put all Matoran in Matoran Spheres and wake them up without their memory "as their saviour". The Toa Metru caused him problem after problem.
Indeed. He widens his scope of ambition, and a new plan, the grand plan, comes to fruition within his mind.
Indeed, though less pertinent to the original question
Right on, dude