Aside from the OP's case, I am wondering in general about this and if you have an opinion readily.
Do you think there is an existing FAA opinion letter or a case pertaining to the situation where an applicant had charges, successfully went through a diversion, and the record was expunged? Does that applicant have to disclose that as a conviction?
What about when the applicant was convicted but it was expunged in a state that provides for complete expungement? (I know expungement on the Federal level is quite limited).