My 2008 (black! GT/CS!) convertible is easy to drive. If you've had several people try it and agree yours is hard to drive, maybe you should approach it from the other direction: drive several 2008 Mustangs with similar equipment and see if they are all hard to drive in the same way. Then at least you will know if yours is out of the ordinary and your attempts should be directed at the car itself.
I have two very comparable cars, and it's amazing how different they handle and drive, even though they are practically identical as to basic setup and mods. Could just be an individual difference thing, with all the tolerances going one way in yours.
My daughter's Plymouth Satellite drove the way you describe, sort of; seemed like sometimes it had power steering, sometimes not. It was a failing power steering pump. Even a 25,000-mile-old Mustang shouldn't have that problem, but not every part on every car is perfect from the factory. If the dealer checks it all out and can't find what is responsible, maybe a specialist could. I'd expect erratic steering behavior not related to alignment would be a warranty repair item.
The wider-tire syndrome mentioned above - car wants to follow lines parallel to the direction of travel, or dart under mild or moderate braking, may just be something you'll have to accept.
