I am so sorry your horse DIva has this problem, and I understand your worries and concerns.
Rotating multi-limb lameness is very difficult to assess, interpret and diagnose and is very frustrating.
Navicular disease is an incurable condition and requires continued management of the pain produced by the disease. I have treated many horses with this condition and many of them live long happy lives with varying degrees of exercise being possible, none have resorted to surgical techniques to improve their quality of life.
Firstly, I would look to try and confirm the presence of navicular disease and I would look to use nerve blocks to localise the area of pain causing the lameness. This involves the injection of local anaesthetic around nerves and into joints to numb them. If the lameness disappears following a particular block, the point of pain/inflammation can be confidently isolated to this area.
The most significant aspect to managing a horse with navicular disease is pain control, and phenylbutazone is the main stay for this, doses of 2.2mg.kg per day should be used. Isoxsuprine hydrochloride is a drug which opens up the blood vessels in the foot and bones increasing the blood supply and so potentially reducing the erosion of the navicular bone through the disease process. This has a variable effect in horses.
Appropriate hoof/foot balance and conformation is vital and so ensuring the toes are not too long and the heels aren't too long is imperative. Egg bar shoes are the most commonly used shoe for palmar foot pain but can be seated to far forward which prevents sufficient support of the heel, this will encourage the strains on the navicular bone which causes the navicular disease process to develop.
Surgical procedures are possible as you mention and nerve cutting is relatively commonly performed, but does present with ethical considerations. However, another possible procedure is surgical release of the navicular ligament, this releases the tension across the bone and can help slow the disease down. Most effect has been observed in younger horses, early on in the disease cause with minimal x-ray evidence of the disease.
It is worth repeating the x-rays at a later date to identify bone changes as this is a characteristic finding in navicular disease.
Importantly, horses should be encouraged to move and exercise unless they are very sore. This movement promotes blood flow to the hoof and foot and also avoids fluid swelling in the lower limbs.
So to recap, get the diagnosis confirmed (nerve blocks/repeat radiographs) continue phenylbutazone and consider isoxsuprine use and ensure foot confirmation and shoeing are adequate.
I hope this answer is helpful and I wish you and your horse all the very best.
If you have any further questions please do not hesitate to ask me
Dr Dan Makin