Firstly Ethan and ponyface, a drought/arid dry environment will be doing your horse's hooves GOOD! It is highly unlikely that they will be suffering from being too dry. You don't need to 'hydrate' a horse's feet(except to make trimming easier!
) & topical goops, especially oil based, aren't helpful from a health perspective & don't prevent/cure cracks. What is likely to suffer from drought, which can affect a horse's feet is his nutrition.
The heels should be level with the frog as far as height.
The bars should be close to level with the sole.
The wall should be close to level with the sole, also.
Yes to all of the above.... generally. There are lots of 'conditions & exceptions tho. Eg. if heels are high &/or heels are weak, they shouldn't be trimmed too short, particularly never into the sole plane. Walls in soft/yielding environments can afford to be a bit longer & this can be helpful for grip, whereas walls in hard environs can't really afford to protrude from the sole plane at all.
The length of the crevice between the heel and the base of the frog extended to the point of the frog, should equal the length from the tip of the frog to the toe.
Not sure I understand but if I do, no. The frog, from the back of the central sulcus to the apex/tip should be at least 2/3 the length of the entire hoof capsule - so there should be 1/3 or less in front of the frog. Equine Lameness Prevention Organization
explains the whys & wherefores of a/p balance quite well.
Also, The process of bringing back underrun heels?
Ensuring toes are kept back is the main thing. Frog support pads(NOT under walls) can be helpful too, to allow crushed heel walls some relief.