Have you done some study into hoof anatomy, function, health factors? If not, I'd suggest hoofrehab.com as a good start.
Low sugar/starch diet is a huge one. No grain or molasses. While horse's diets are generally lacking in a range of nutrients, it depends what they're getting as to what they may be lacking. I suggest doing at least a basic diet analysis(FeedXL.com is a great service) before choosing supps. A good complete supp is often better than a hoof-specific one.
Hooves do need to be fed from the inside and I agree with the person that said hoof conditioners are basically just cosmetic. They may make the hooves look prettier, but putting goop on the outside does nothing much for the health of them. Also if it's oil based goop, hooves are meant to 'breathe' so this can cause them to dry out more - despite how they may look when oiled.
Growth not only depends on diet, but on the state the hooves are in - eg if they're laminitic, they may grow slower or quicker than normal. If they are contracted/constricted, they will grow slower. Generally speaking, miles are a more accurate measurement for hoof growth than time, as the more exercise(assuming good hoof function), the quicker they grow. I've heard of bare endurance horses who grow their feet out in 4-5 months.
Regarding regular trimming & letting her heels grow a bit, of course without pictures & more info(which you can post for more specific advice), I don't know for sure, but what I find this generally means is that the horse has either got *nice* short heels(they're meant to be short), or, especially when combined with long toes, the horse has in fact got quite long heel walls, but they've been crushed under and the horse is walking on what should be the vertical part of them, so has no heel *height*. Generally keeping the toes backed up and quarters short, along with preventing the heels getting longer tend to help the heels become upright again, and they can gradually be lowered to a good height.