That's quite a situation you have there. I'd collect any and everything
you have to prove you filled out and sent in your B2 application in a
timely manner. Copies of the FedEx receipt, the money order receipt,
and any other receipts. You made photocopies of the application and
supporting documents before sending it in, right? Show them those as
well. Also, a few notarized affidavits from people that know or live
with you that you did send in the application wouldn't hurt. No fact
that would help others to believe that you sent in the application in a
timely manner is too small.
You can stay until August 22 if that's what
they said. I wouldn't leave before you try to prove your case to the
officials. But don't delay in doing so.
Also, realize that if the
USCIS does accept your case and doesn't hold this against you, they are
being quite merciful to you. I have no other adivce for you.