Which Is Enclosed Herewith?

Herewith means with this document, text, or book. You can use herewith in a letter to say that you are enclosing something with it.

Which is correct attached herewith or herewith attached?

Herewith means attached. Do not use both. In fact, do not use herewith.

Is it Enclosed are or enclosed is?

Always use 'are' with 'criteria'. Definitely it would be "Enclosed is an orginal and one copy of the document." More than one would be "Enclosed are original copies of the document."