The bond is grossly over inflated, what choice do you have in signing it when you bring your family half way across the world and have resigned from your previous job.
Sorry but that's BS. You were told at your interview that if you were not type rated, then you would sign a bond. If you did not want to sign an 'inflated' bond, then why did you? Were you forced to sign a bond? I don't think so. You wanted the job, you wanted the type rating or whatever, and yes, you signed the bond, sorry but it is your responsibility to check what you sign is it not?
I personally do not like bonds, however they have become a fact of life in our business and as a responsible adult you have the choice of whether to sign or not. You also have the responsibility of settling your account if you decide to break the bond or contract.
The letter that Etihad wants is simply to see if you are going to play the same game with them, I would think.