Hum. Well, I guess that just brings up even mroe questions, but this is just me thinking aloud, not expecting any actual answers:
(cut for later)
And so forth and so on. Again, not expecting answers. The more the second trilogy is thought about, the more Lucas manages to piss me off. It's more like he was riding a crashing X-Wing into a planet while shouting in excitment and waving about a hat.
This may not be the entire answers, just what I can remember.
* If a structured rule is the idea, why does it have to be forced on everyone?
The Sith tend to hold that their way is the only way that should be allowed. Add to this greed and the will to control everything that was once the Republic and you have an Empire that spans about two-thirds of the galaxy.
* Why do they have to attain it constantly with murder and domination?
Because they don't have enough forces to rule everything directly and orderly...so they use fear instead.
* Why can't it be a peaceful coallition?
"What are you some sort of Jedi?"
Actually it is because the Empire rises out of the ashes of the Republic and the Clone Wars. They spend the first decade or so suppressing the former Separatist Worlds and occupying them until such time as they fully submit to the Emperor's rule and the New Order. There are also those worlds that harbored Jedi or supported the Jedi, which are considered enemies of the Empire and traitors. They are hunted down and those that helped them punished harshly. Then the rebellions start and the oppression continues.
* Why can't worlds that don't want to be a part of it, not be a part of it?
Because that was what the Clone Wars was supposedly fought over. The galaxy cannot afford to be divided as it causes conflict and chaos, thus the whole must be made one and orderly. No matter how many suffer and die to gain that order. Or at least that is what we are lead to believe by the Empire.
* Why does it have to encompass so many worlds across great distances? Why can't that structured rule be in a certain area?
Again because of the Clone War and the situation that lead up to it. The galaxy must be whole and controlled to prevent more galaxy spanning wars, as iron rule will end all war. Or it will once everyone is so fearful of the consequences of defying the Empire that no one would dare going against it (hence the Death Star).
* Why after so much failure, murder, and coercion, do they not realize the way they've gone about it for so long hasn't cease chaos but created it and structured something worse than the Senate could have ever done?
The Empire was to rule through fear when it could not rule through other means. The Rebellion against the Empire was relatively small and considered a nuisance to the Empire compared to the Separatist movement from the Clone Wars. the Rebels were something to be hunted down and slaughtered. Once that was done and a Death Star was in service, the rule through fear would be complete and no one would dare oppose the Empire. Or at least that was the theory.
* What if the Empire decides they don't need the Sith? What do the Sith do then? Do they have the numbers and power to force their continued rule?
The Emperor designed his empire to not be able to function without him. Remove him and the military governors and high admirals and generals will fight for control and the Empire will implode. The Emperor has enough people loyal to him, and him alone, to ensure his continued rule. He by himself is extremely powerful, not only in the Force but with his ability to manipulate people to do his bidding even if they don't realize they are working for him. Should the Empire decide they don't need Palpatine and do kill him, the Emperor set up plenty of contingency plans to make sure they cannot have his Empire (Scorched Earth policies). Several of these started to happen after his actual death at Endor, which the New Republic had to deal with. The Empire fell on itself, with what remained heading into the Unknown Regions for reasons unknown (we will likely find out more in The Last Jedi or Episode IX)
There is potentially other reasons for Palpatine to rule the way his did and for his massive military expansion following the Clone Wars, but that information has not come out. Only hints that there was something important on Jakku and in the Unknown Regions that caused Palpatine to be concerned.
Vader is useful to the Emperor, but is not necessary for the Empire to survive. He can be replaced.
