You have great writing and communication skills, just to have written that.
I can somewhat relate to your problem. I have not had a directly abusive father, but my father's way of controlling was a bit different. He played on the emotional strings, and becoming very silent and passive-aggressive, to make me feel bad if i ever argued against him. And he has a hard time admitting mistakes, even if i knew i was right. But this has made me become very self reflective and aware of my direction in life, regarding the people i want to have around me. But it has also left it's marks.
That is why i would advice you the same i advice myself - you must break free. This guy has lived 20-30 years longer than you have. That is not to say he is more intelligent or a remotely better person, maybe in some areas he is better than you than other areas, and you are better than him in yet others. But he has been around people longer, and he has had you under his "wing" for your entire childhood. He knows you better than anyone else. Therefore it is best for your to stay away, and walk your own path, if you're ever going to feel true self esteem. Because otherwise, you will feel that you give him your power, and that is the wrong way of living.
How do you see yourself 20-30 years from now if you decide to work for your father, even if you become financially stable and secure?
What would you do if you won the lottery, would you work for your father then? If not, the answer must be to find another, better paying job, if that is your main goal right now.
A true leader is someone that people want to follow, not someone that whips someone's back until they follow them. This requires insight and self-reflection, and it seems to me you have more of it than your father. Otherwise you wouldn't feel this way.
Not trying to make your father out to be the bad guy here, he is obviously a man of the law, and of good -intentions-, but the way he goes about doing things is not to care about privet and personal feelings. He should be honoring your word as much as he demands it from you. And since he went about and checked your credit and paid the bills without asking, that is a major invasion of privacy. That is another red flag for you.
I recommend keeping a healthy distance and only answering him peripherally. Do not engange in emotional issues. This is your only way of "sanctioning" him without inducing guilt to yourself in the process. Arguing will never bring anything to the table.
It is the same strategy countries and presidents use when other countries behave "badly", even if they have less resources and a smaller military unit. You can implement sanctions, and keep developing your own life as you should do.
And on a side note. This is a bit on the ethical edge of things, but i'll bring it up anyway:
It seems to me like your father is quite successful in life. He has made a bit of money and wealth. And it does seem to me like you are a hard working individual. You are making money, just, even keeping 2 jobs.
One day, your father will pass away. Then his wealth and riches is -legally- going to be passed onto you, if he has not made any changes in the will.
So therefore, it is in your best interest, economically, to not break all ties with him and "going to war" with him. This is part of why i want you to keep a healthy distance.
But not only economically, but also for his sake. He has after all, in my opinion, your best intentions at hand. But the way he goes about and does it, is wrong. Therefore you must keep the distance, but not go to war.
That 20 000$ payment, you can take into perspective. Imagine if some day you will become a financial success. Imagine if you won the lottery, would those 20 000$ mean so much then? You could just whip 20 grand into an envelope and put it in his mailbox. So keep things in perspective. Once day it will not matter so much as it does today.
But this is your life. Only you know your true limits. What i want is for you to live with true self-esteem, and not feel like your father is dominating your emotions. Because that is a very dangerous way to live life. It is not worth any sum of money or any kind of lifestyle. It can drive you into complete depression and then you will not be able to do anything, anyway, even make you do irrational actions, especially when it comes to drugs and intoxicants. I know this from personal experience. Just give yourself some space, do NOT go and work for him. Be strong enough to walk your own path, that is the ultimate sign to him that you are living free.
Sorry for the lengthy answer.