3. He didn’t know
The one big way that Anakin’s sacrifice would be cheapened is if he knew that throwing Palpatine down the lightning shaft wouldn’t destroy him forever. Perhaps he was doing it to throw everyone off the scent, knowing that years down the road, Palpatine, who would be assumed dead, would be able to live in the shadows and manipulate a new evil empire.
That isn’t the case though. Anakin thought that throwing Palpatine down the tube would kill him for good. His sacrifice was noble, it was the final shift from evil to good, and it saved not only his son, but the galaxy as well.
What ended up coming after has no bearing on the merits of Anakin’s decision. If you ace a written test that you didn’t study for, do you get an asterisk by that ‘A’ that says “*ill-prepared, lucky”?
No, you get an A. Good for you. Anakin gets an A for his efforts. Maybe he should have known that Palpatine would come back, but what would he have done otherwise, not kill him?
He did what he had to do for the sake of the galaxy and his family. You can’t cheapen that in any way.
Onwards to No. 2.