1) Is it possible to change the "Created By" value on a document to a
different user? (Through the content database or other means?)

For example, I would like to change the "Created By" on TestDoc.doc
(stored in the document library TestDocumentLibrary) from DO\User1 to
DO\User2)

2) If not, is it possible to hide the "Created at" from showing up at
the bottom of the page when a user right clicks on a document in a
document library and selects "edit properties".

(I realize that you can specify which columns to show in the document
library view, but that doesn't prevent "Created at xx/xx/xxxx 00:00
by NAME" from showing up at the bottom of the page when you edit the
document's properties)

3) Thirdly, what happens if the AD account (that created the
document) is removed?