Hereby is used sometimes in formal, official declarations and statements to give greater force to the speaker' or the writer's affirmation. People will say it sometimes to emphasise their sincerity and correctness.
If you wish to be associated with a particular high risk and/or high profile situation and benefit from the rewards of that association, you have to accept the consequences if things go wrong - you cannot dissociate yourself.
'If you will' is used as a way of making a concession in a sentence:
He wasn't a very honest person, a liar if you will.
Here, it is used a way of accepting that the reader or listener might think of the person as a liar, but without commit the writer or speaker to that position fully.
If you say something in all honesty, you are telling the complete truth. It can be used as a way of introducing a negative opinion whilst trying to be polite; in all honesty, I have to say that I wasn't very impressed.