I was faced with a very similar situation growing up, as were my 2 siblings.
Unfortunately there is not much you can do for your son 21 years of age, as he is an adult and can now choose who to live with. I would say to tell him about your concerns, and suggest that a change in his environment may be beneficial for him, whether that's coming to live with you or living on his own at an apartment. Try to work with him, and make a deal if you can. This would be the best way to get him to at least look into leaving his father.
As for your 16 year old, since you do have custody of him you should be able to tell him to move back with you, however due to his age he may have the legal system on his side if he has a substantial reason to remain with his father. Since your Ex was abusive and an alcoholic you should have a substantial case for the legal system to have them rule in favor of your son living with you. In addition to the legal matters, I would suggest trying to get more involved in his life, maybe teach him how to drive, go out to the movies, fun stuff for teens. And once he builds trust with you, perhaps you can ask him to move back to live with his younger brother.
I'm glad you were able to get your youngest to come live with you, and he goes to show that progress is being made in the situation. Spend time with your youngest, as this can be a very stressful situation for him. He needs someone to be there for him through all of this, as it is a lot for children to deal with.
As for the boys defending their father, let it be known that this is completely normal. It happens in a lot of families who have children and divorce. Frequently the children will take the side of one parent, and defend them whenever they feel the integrity of their parent is being attacked. I assure you that this can change in time, so don't feel worried by it.
As for the magical aspect, just keep trying. It seems like you're doing it all very well, and you should maintain your efforts. Never give up or stop trying. It may take a while, but you'll eventually see the effects of your hard work and dedication.
I hope this has been of help to you, if you need anything else please don't hesitate to contact me. I know what you're going through, and I wish to help the best I can. Take care, and I hope for the best!