There are thousands of religions in SW. Certainly human believes varies wildly.
The Empire? From all we could muster the official religion is no existant, probably it inherited the laicism of the Republic. That or it was openly atheist, since it's propaganda dismissed the Force as a superstition.
>That said, how does Sheev & Vader get away with being Dark Side users?
Palpatine wasn't publicy know as a Force user. Vader shenanigans were restricted to a small size of the navy/army. People even had doubts if Vader was a droid or something. He was just the mysterious guy.
We're talking about a galaxy of quintillions. Coruscant alone was supposed to have a trillion people, the Jedi, the most famous and active force using group was numbered in 10k. So people actively knowing about the jedi, dealing with them, seeing them was very low, that's why it was easy to the Empire dismiss them and the Force as myth in the short span of 20 years
>Word would get out, even if only through the military ranks. How does the Empire explain that "Every Jedi except this scary guy dressed in black were EVUL."
People there Vader was somekind of fallen Jedi or similar who sided with the Empire around the end of Clone Wars. Period. Also the Jedi were gone just like that. Poof! Tarkin considered Vader an antique, the last remnant of their religion ("The Jedi are extinct, their fire has gone out of the universe. You, my friend, are all that's left of their religion").