i have come late (very late) to this post and I am only responding to some of the questions posed on why is hair ‘ervah’,(see SJSin NYC)etc..
Actually, the word ‘ervah’ is used for other parts of the body (“shoik”, thigh) and also to the voice of a woman. there are some kabbalsitic aspects to the hair of the head being the soruce of the ‘soton’ but I’ll refrain from quoting these mystic opinions.
Basically, hair is considered ‘ervah” because it was and is a source of beauty for women and hence, a source of possible seduction from a forbidden woman.This is clearly the reason as it is equated to the reason for “shoik” (thigh) and singing being an ‘ervah”. There is no intrinsic issur on hair. This is clear as girls who are unmarried are not subject to this rule. Additionally, the Aruch Hashulchan allows to say “divrei kedusha” in front of a woman with uncovered hair. He decries this custom but BECAUSE of this fact ,he says that uncovered is not an ‘ervah’ as far as kriat sheman is concerned. Rav Moshe Feinstein zzl’ also allows a “tefach” of hair to be uncovered as the gemoro says ” tefach be’isho ervah”. Clearly, hair is not intrinsically ossur but it is the allure of the hair that is the problem here.
What has happened is that woman’s hair has been imbued with some supernatural issur,based on kabbalistical sources and therefore, sheitels became OK because they were not the real hair,
In truth, all the beautiful sheitels are an end=run over the fact of uncovered hair. It would be a lot better to have a hat or a snood or a scarf. However, because women have adopted sheitels enthusiastically and the rabbonim will not issue a gezeira that will not be heeded,they have refrained from doing that. Hence, sheitels are here to stay!