The purpose of nailing a horseshoe over a door was placing Iron over / upon the door. Iron was the bane of fae and magic. The cross, on the other hand, was the bane of evil spirits, and in christianized areas was often seen as being as good or (perhaps) better than iron. Of course, a blest iron cross would likely be covering both areas of thought / belief. Although I have not heard of such I would not be surprised if such ever existed.
As for silver and werewolves, most areas did not make much of a distinction between werewolves and vampires. Both turned into wolves, both attacked the living, both were viewed as fiendish in some manner. Gold was the color of the sun, the color of day light. Silver was the color of the moon, the color of light in the night, in the darkness. Of the two, only silver was worthwhile as a weapon material. Silver was suggested for weapons as it was believed that vamps / weres could swiftly heal from any non-mortal wound (such as beheading) unless the weapon inflicting it was blest or silver.
Prior to hollywood I don't think there was any referrence to 'silver bullets' for slaying werewolves, but silver knives were certainly suggested and even forged for such a purpose. In many of the stories in which a paw is chopped off only to become a hand, it was a silver (or blest) weapon that removed it - and allowed it to return to its normal form - revealing the true (were) nature of the beast. Except in some stories silver did not see much use, however, due to the lack of werewolves.
Iron was more interesting. It was not uncommon for some rural areas to have nuggets of iron along the border of their farmstead - to keep the fae out; to have horseshoes or other large bits of iron over doorways or upon doors for the same reason. In some areas it was believed any iron would have such an effect. In other areas only cold iron - raw iron forged without the heat of a forge - would do (due to being more 'natural', I guess).
