What you have described is well within the standards for child abuse. I am the first to encourage disciplining a child, even spanking, but this should never be done in anger. What I tell my children is that if they are angry, they should not even touch the other person. An adult should know better. By your own accounts, she has twice now inflicted physical harm on the child - a dislocated elbow is really bad, and I am tempted to think that the fingernails digging in was no accident, but accident or not, this is clearly physical abuse.
Further, the yelling and personal attacks that you describe are well within the bounds of emotional abuse. In our house, we have some rules. One of them is "never compare one person to another", and another is "never make 'you are' type statements." You can criticize what a person did, and say how it hurt you or why you think the action is wrong, but the minute you say "you are..." you are attacking their identity, the core of their being, not just their actions. This is abusive and wrong.
You can rest assured that the wounds and scars your wife has inflicted on your child will haunt him for the rest of his life. You need to stop this.
I would strongly advise you to report all this to child protective services. Record one of the shouting matches and take this to the authorities before your child suffers further damage. Report to them the instances of physical abuse. You must have medical records from the time his elbow was dislocated. If it was just the shouting matches, I might have advised differently, but this is obviously out of control and will only get worse.
Together with the authorities, you can put a lot more pressure on your wife to get some help. Either she stops this and gets treatment for whatever problems she is having, or she will lose the kids - with proof of child abuse, it is very basically impossible that she would be given custody of the children. But, be ready to be there for your wife and help her through this. With help, she could come out of this a better, stronger woman, with more peace than she ever thought possible.
I know that going to CPS is very scary - I have been there, and it was super-scary, even though my wife was not the abuser. I cannot imagine what it is like for you. You have a tough road ahead, but you must move to protect your child immediately.