While I can only speculate with regards to the first part of your question, I can perhaps help with the latter.

Basically, the practice of protecting the feet of horses was not universal even among the Greeks and Romans. Fabretti, an Italian antiquary, examined with care the representations of horses on many ancient columns and marbles, and found but one instance in which the horse appeared to be shod; and in most specimens of ancient art the iron horse-shoe is conspicuous by its absence. However, in the mosaic portraying the battle of Issus—which was unearthed at Pompeii in 1831 and which is now in the Naples Museum (go, if you ever have a chance)—is the figure of a horse whose feet appear to be shod with iron shoes similar to those in modern use. There's also an ancient Finnish incantation against the plague, quoted in Lenormant's "Chaldean Magic and Sorcery," which states:

"O Scourge depart; Plague, take thy flight. . . . I will give thee a horse with which to escape, whose shoes shall not slide on ice, nor whose feet slip on the rocks."

Winckelmann, the Prussian art historian, describes an antique engraved stone representing a man holding up a horse's foot, while an assistant, kneeling, fastens on a shoe. In the works of the Roman poet Catullus occurs the simile of the iron shoe of a mule sticking in the mire. More contemporary historians relate that the Emperor Nero caused his mules to be shod with silver, while golden shoes adorned the feet of the mules belonging to the notorious Empress Poppaea.

According to N. S. Shaler, iron horse-shoes were invented in the 4th century in Greece. (It's interesting how he decides on this but I won't go into that here.) But even France in the 9th century, horses were shod with iron on special occasions only. The early Britons, Saxons, and Danes do not appear to have had much knowledge of ferriery.

Therefore, the modern art of shoeing horses is thought to have been generally introduced in England by the Normans under William the Conqueror. Henry de Ferrars, who accompanied that monarch, is believed to have received his surname because he was entrusted with the inspection of the farriers; and the coat-of-arms of his descendants still bears six horse-shoes.

digifox appears to have you covered on the dating of horseshoes. In terms of horses as a work animal, until the advent of the Horse Collar horses weren't super useful for tasks such as pulling plows, carts etc, as they would choke. In Europe this takes a fairly long time to develop (less time in Asia). Until this point oxen are the main source of animal labor. Check out an oldy but a goodie, Lynn White's Technology and Social Change for some info on horse technology (collars, stirrups etc.) in Europe.

It should also be noted that iron and horses were highly valued commodites in early medieval Europe, which probably made owning or shoeing a horse an expensive commodity unless you were an aristocrat.