Okay, putting this thread back on topic.
The question is: How far back does Sidious's plan go? The Answer:
Palpatine made his way into Naboo politics in the beginning of his mid-life, then made his way to Coruscant as an elected representative. As time went by, he continued to manipulate people until he was elected Supreme Chancellor, then made himself Emperor. It was a simple plan, and every politician does the same thing. The only difference is that Palpy wears a black hat while doing it.
From the beginning of all of this, he had been a Sith Lord. His plan began when he was a young man, which was not 800 years ago. He is a regular human being, and his normality is what made him want to control the galaxy. His plan goes back only a few decade before TPM. There is nothing more to it.
If you don't like that, and would rather think of the situation as being as massive conspiracy of Sith ghosts and stuff, then you are neglecting the psychology of how evil is born.
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Corn.