But USCIS told me they have no way of knowing if a letter has been mailed out - only that it has been returned undelivered. Do you know for a fact that what I have been told is incorrect?
If you speak to the 1st level of USCIS customer support they have no way of seeing if a letter has been sent out. You must request your call to be "escalated" or transfered and then speak to an IO who will be able to tell you if a letter has been sent out. An alternative to this is to request an Infopass which will allow you to talk to an IO at your DO.