Congrats on your progress, strength to work through it, and remembering your behind the scenes support (everyone in here). Your note seems full of strength and power, so I am truly proud of you.
Just remember that things don't always go the way we planned. Be prepared for any set back. If the creep is there, don't try to force the strength to show your are over it, beyond it, or being back in victim land. Leave if you feel the need, no excuses need to be given, just get out. If your dad or others are not supportive, don't argue, lash out, threaten, and definitely don't cry. The tears can come when your are with those who are on your side. Now if dad and others do back you, then yes, tears are fine. Good old cleansing tears.
One technique I give myself, my kids, and others when dealing with stressful situations where you are afraid of saying the wrong things, responding out of hate/anger/frustration. The technique is to place the tip of your tongue on the roof of your mouth, just behind your upper teeth. And the more upset, the more you want to respond, push harder. It is hard to talk with your tongue occupied, and this can help you to focus on the task at hand - protect yourself, protect the hurt little you that is inside you, wanting to lash out, and it gives your mind cues to analyze the situation. I'm sort of rambling at the moment...a horribly long week, and I'm back in my mode of wanting to open up a house for my kids, a place where they are safe, getting intense therapy, learning appropriate behaviors, and able to laugh. Oh how I wish Bill Gates would adopt me, or at least bank roll my dream.
But anyhow, back to you...I just want you to be proud of yourself and your growth, just be ready for the others to not be so thrilled, to be angry that the boat is being rocked. Maybe...instead of giving your dad the ultimatum: him or me...just say something like "dad, I love you and everyone who will be there, but I just can't be around him. I don't want to spoil the holidays, so maybe you and me and whoever else you want, can meet a day or so before or after, to get together." Let him make the decision and his decision will help you know where you stand. No one like ultimatums, and will often react in just the opposite way of what you want, just out of a reaction, not thinking. But giving him the option, you are putting the ball in his court, and you are showing the strength that you want the holidays to progress, with or without you. Give him choices and hopefully, he will choose right. And also, it took you time to get to a position of strength, give your dad some time. He may not give the answer you want right now...but after he processes, thinking about it, and has time, he will likely come around. It was his brother, and it will be a lot for him to deal with. Now here is another hard part. Lets say he says/does something stupid and hurtful. Take that as your cue to leave. But leave the emotional door open. Like I said before, he too will need time to process all this, and fear make people say and do the most stupid things. When he comes around, don't force an apology or big peace offerings. Him calling or coming by may be his way of asking for your forgiveness. And that will also show that he has dealt with it and does believe you and wants to help. A lot of men just don't know how to deal with these issues, so he may just be acting out of hurt and fear.
I hope this is some help and make sense and yes, you can write, I will try to be more concise and coherent.
Keep us posted