Really, Palpatine took control legally... he manipulated a weak-minded fool into convincing the senate to give him emergency powers. After that, he could do whatever he wanted, without consulting the Senate... at the moment he was given those powers, the Senate became nothing but a figurehead. And I believe that Palpatine has always been a Sith... Maul had been training practically since he was born to be a Sith Lord, so why not Palpatine? I think that since Bane's army was defeated, and the Sith were laid low, they had been trying to infiltrate any high position they could find... until they made it to the office of Supreme Chancellor 1000 years later.
As for the Empire... the Empire was really just the Republic under martial law. Once Palpatine was deposed, and martial law ceased, the surviving members of the Senate could reassume control with little difficulty, especially since they obviously had the people behind them.