As far as proud flesh is concerned, granulation is good and a sign of the healing process. Excess granulation is proud flesh and too much is not good, unless you don't care about lumpy hairless scars. Proud flesh can come and go quickly but pressure bandages slow it down and you know it's a problem when the center of the proud flesh is above the level of the skin and wrapping doesn't help. I agree Vetericyn is great stuff although I've never had a blown abscess to use it on.
For your horse's coronet band, and the crack that's there now, I agree that it's likely that there will always be a crack growing down from that old injury. However, since it's still healing, you might want to get a really good farrier out to look at it and see if there is some corrective shoeing or protective trim/shoeing that can be done to unweight or redistribute the pressure on that hoof wall. If you could get that now, while it's still healing and growing out, you might be able to have fewer problems in the future.
For example, our gelding has a hoof crack from an old coronet injury that he got before we bought him. From time to time he'll split from the ground up and needs trimming to keep the crack from widening and deepening. This fall he needed corrective shoeing because he broke a chunk off. But, we've had him for 5-6 years and this is only the second time it's been anything we had to really worry about. Having said that, we're scrupulous about regular trims every 6 weeks and we have a great farrier, and the horse gets shoes when we're trail riding on rocky trails in the summer, so that we don't damage the hoof.