I think once you get the tires right, you'll be in a good place. Then if you want to do more mods, you will feel afterwards what did what and you will feel more comfortable.
Again, I would buy new front tires to match the brand/model you have on the rear and start there. There's a good chance it's behaving weird because of that. Don't throw more parts at it until you do that. I know it may seem that you're closer to getting where you want by throwing more parts at...
Well, the stiffer you make the suspension and lower the ride height/increase the negative camber, it's probably going to tramline MORE not less. It's a feature of the car. These Mustangs tend to tramline. New owners of stock GT350/R have complained about it. It is just what they do. Personally...
Good you found that. At the least your car will handle better and more tight with those bearings. I've got them on my car. Let me know how you get on. That's a good call on matching tires imo. For a car like this, you want it to handle right. Having matching tires on all 4 will help that a lot...
I don't think the bearings are going to help matters unless a part was loose or damaged. Even so, in such case I would expect you'd be speaking of different problems. My money is on the tires first, alignment second.
That could be contributing to it. Different tires with different grip levels and wear levels, as well different behavioral characteristics. These cars are also very susceptible to tramlining so I wouldn't be surprised if that's it.