Hi Nina ~
I had a mare with a wire cut in the same area which looked very similar to Atreyu’s. Because the coronet tissue was damaged, there was always a “line” where the hoof grew down but because the overall hoof was healthy, once the initial injury healed it was only a scar, never an open crack.
Make sure your hoof supplement has good levels of copper and zinc, which are important for healthy hooves, in addition to the biotin. Something like Source Focus Hoof https://www.smartpakequine.com/focus-hf-133p would be a good supplement along with your Horseshine; the Focus Hoof supplies iodine and a good level of methionine along with biotin.
A good “hoof” supplement is, for the most part, also a good general supplement.
As long as the underlying structure of the hoof is healthy, your horse will probably be fine with just boots to protect the foot on rocky ground or gravel.
Patti Woodbury Kuvik
in Vail AZ