Is it possible to set an encryption password different than lock screen pin?
Please, keep in mind -- one problem, one question here. Two questions in one post is a bad idea.

Now:


first question: take a look here or here or in many other places,
second question: no, encryption is irreversible!


Once applied the only way to disable it / change password is to do factory reset (loosing all data / wiping phone memory -- as you write) and perform another phone encryption after that.