Hmm, well, a divot in the coffin bone MAY cause a weak spot in the toe, but it will create a divot where the sole and wall meet at the toe. He's wrong about the navicular bone.
If it's usually humid in your area, fungus may be a cause in your case, but proper trimming can sometimes take care of that, as fungi are opportunistic, in so much that a weakness may already exist from poor hoof balance, a divot in the bone, as mentioned or excess moisture. Soaking with Apple Cider vinegar or White LIghtning (a medicine for white line disease) etc and good trimming can grow it out. Opening the hoof wall further can sometimes just make it worse, as it irritates hoof structures, making them more suseptable, and for that reason, I'm not a big fan of epoxy to cover it, either.
You're right that an upright foot can promote a toe crack, and it's telling you the hoof isnt' balanced right. Even hooves that seem to be prone to being upright, usually are ready to have the heels lowered some if thy are cracking all to heck.
Probably isn't a dietary thing, unless his other hooves all are of poor quality (shelly, flaky, cracking, etc). Moisturizers and oily dressings can make it worse by trapping fungus in the wall, so I'd recommend skipping that. Brings us back tot he anti-fungal treatment and good trimming, which I doubt he got a good trim if that guy was a quack.