If a green horse travels on their forehand because they're green, and haven't yet learned how to carry themselves under a rider, well, then yes, training and conditioning will help them find a more balanced way of going.
If a horse is built downhill, there is nothing you can do to change their basic conformation. You can *influence* their way of going under saddle, by asking them to push off their hind end and use themselves more correctly, but you will always be working against their conformation while doing so.
There are several excellent threads and articles on this board on collection, what it is and what it isn't, and ways in which the word is commonly misused. I'd suggest taking a look at those for more info.
The ciruclarish rubs you describe sound like the beginning of saddle sores from a badly fitting saddle.