Die Haare is the plural of das Haar. If you speak of a hair style you would normally use the plural (different from English). "Look at her hair." (not hairs) "Sieh dir ihre Haare an."
You can, however, also use the singular "das Haar" to refer to all the hair on one person's head. This would sound a more literary.
Langes Haar ist wieder in. (singular)
No, "in" takes the end position because it is a predicative adjective, so the verb consists of copula verb "ist" and predicative adjective "in" and is pulled apart. That's how German grammar likes ist.
"The Germans have an inhuman way of cutting up their verbs. Now a verb has a hard time enough of it in this world when it's all together. It's downright inhuman to split it up. But that's just what those Germans do. They take part of a verb and put it down here, like a stake, and they take the other part of it and put it away over yonder like another stake, and between these two limits they just shovel in German." - Mark Twain's Speeches, "Disappearance of Literature"
Because English usually uses the singular "hair" as a collective noun for all of the hair on one person's head, while German can use either singular "Haar" or plural "Haare" -- and in this sentence, used the plural.
Translating it into "Long hairs are ..." would not be idiomatic in English; it's not how we express things.
I wrote "long hair is hip again". It was not approved. Mistakenly so, I reckon.