So get a certification from the arresting agency that your record is clear . . .
You seem to be making this too complicated. They cay you have a conviction. You do not.
Get a piece of paper with a court seal that you were not convicted when you were arrested-- or were "discharged" from the court without conviction. US CBP isn't really going to care about what you say your local precinct says about what should or shouldn't be in the computer.
I have the piece of paper you are talking about and submitted copies to DHS, I have verified at the police station where I was taken when I was arrested that nothing shows on my record...I even asked the station head if there was a "secret" database that is maintained that would still blacklist me (he assured me there is not).
The problem is I have no other method of clearing this up other than DHS since they are a one stop portal for clearing these things up for foreign travellers.
Really all I want to know what is a reasonable timeline for this process to occur? Nothing has changed on my DHS resolution page and there is no one to contact to even speak to a live person, e-mails get sent back with a standard response to date.
I'm going to speak to an immigration lawyer and see what can be done, I really have no re-course than to wait or attempt a waiver application for a charge that doesn't exist...god knows how that will delayed as a result.