How do the pigs handle their violations of the Seven Commandments? How do they get the other animals to accept actions that had once been forbidden?

The pigs are pretty shameless in their violation of the seven commandments. They realize that if they adhere to them too strictly, then their power will be undermined; they will not be able to establish their dictatorship at Manor Farm. The pigs simply go right ahead and break each commandment as and when they think they can get away with it. Then they try to offer up totally unconvincing, self-serving justifications for their blatant transgressions, subtly changing the commandments to suit themselves. This is a way of maintaining the pretense that Manor Farm is still being run on the basis of noble ideals. The pigs want to set up a dictatorship, but they don't want to make it look too obvious.
The most blatant example of the pigs' subversion of the rules comes with their distortion of the sixth commandment, which states that "No animal shall kill any other animal." This is no use whatsoever to Napoleon; if he's going to be dictator, he'll need to crush any animal who gets in his way. So the commandment is subtly changed. Now it reads "No animal shall kill any other animal without cause." And who gets to decide whether or not there's a legitimate cause for an animal being killed? Why, Napoleon, of course.
As the pigs consolidate their dictatorship, the opinions of the other animals on the farm become even less important. With his tireless propagandizing, Squealer does his level best to sell the animals on the shredding of the seven commandments, but it's terror, rather than persuasion, that will be used to keep everyone in line from now on.