99% of any dressagehorse isnt working 100% percent correctly, they all have their little habits or problems. thats why no matter what level in dressage your (not you specifically,everyone) horse is you always have somethign to work on, lol which is what makes dressage fun

i personally like to learn from my trainer, and read books just so that i learn even when im not at the barn.
100% being an exageration, I think you know what I mean. I'd doubt that the horse was consistantly on the outside rein, over it's back etc.
As far as piaffe/passage goes, it's not even so much the fact that it's quite difficult to teach, it's the muscle development required for the horse to be able to physically achieve the movement. Not every horse can piaffe/passage as well, a chunky little QH is going to find it a heck of a lot harder to get this level of collection than for say an andalusian which is bred for collection.
You can teach it on the ground, but again, the muscles required make it so difficult for the horse to support the majority of it's weight on it's hind legs that it's unlikely you're going to get a true piaffe unless the horse is one of those 'freaks' that is build to do it and offers it at a young age.
Dressage certainly isn't a sport for 'tricks' although it make look to be so to onlookers who aren't involved in the sport. It takes years and years of consistant, carefull and skilled training to take a horse to the point where it is able to perform such movements that require a huge degree of collection.