I've never seen a horse walk around supporting most of his weight with his rear end. For a few seconds, in play or in turns, but 99% of the time - no.
If collection was the norm, it wouldn't require training to do it. We would have race horse owners asking, "How can I get my horse off his rump and running?"
In my understanding, it is not collection per se that must be taught, but how to collect and move naturally with the added burden of a rider
in the equation.
Now, I'm no vet, chiropractor, or biomechanics expert, but this is what one of my riding instructors had me do as an illustration.
Consider sitting on your hands and knees. At rest, most of your weight is on your hands, correct? If you tried to keep the same posture and take the support of your hands away, you would fall on your nose. At rest, consider having that weight on your fingertips instead of your knuckles or palms. Pretty uncomfortable. It's even harder if you try to stand still on all fours with your back hollow and your head cranked up and back.
Now, consider moving forward on your hands and knees. Your weight is automatically going to shift back to a degree -- it has to, or you wouldn't be going anywhere at all. You can even move forward on your fingertips pretty easily if you're moving forward.
Try doing all of the above with a little kid sitting on your back -- it's much harder, isn't it? You have to balance yourself and the child, even if the kid isn't trying to hang upside down underneath you. That's when crawling on hands and knees gets harder and the unconscious weight shift becomes more difficult to accomplish and maintain without building up the muscles to do it. Most any parent/big sibling knows how much easier piggyback rides are to give to "good riders" versus wriggling sacks of potatoes.
The horse does the same basic thing all the time. No, it isn't a passage, or a levade, but it is a shift of weight rearward. Collection in the dressage sense is very specific -- dressage riders want to ultimately have the weight evenly distributed across all 4 legs, even entirely over the hinds (levade). In the western riding world, the requirements for what is considered "collection" are less stringent, but the basic concepts are the same. Dressage just takes what happens naturally to an ideal that is required for the horse to do the maneuvers required of the sport at whatever given level. We (general we) all promote a degree of "collection" in riding as a way to allow the horse to carry us more comfortably for a longer time, and also to be as athletic as they can be at liberty (even when they aren't "collected" in the dressage sense of their own accord) at the rider's will and his/her added weight.
Eolith, I don't see any reason why dabbling in Western would do any harm to your horse's posture or progress with traveling less on the forehand. Just keep similar considerations in mind regarding her posture as you do in English tack; encourage her to stay round, forward, moving with impulsion, responsive to the aids, etc.