I don't know the answer to your question but I do know this, like I said before the cases for narcotics are a little more complex because 1) They ask you for each time you were arrested, period. And you have to tell them that unless you have a legitimate reason to leave that out of your application. 2) With narcotics cases the first level(before BIA and circuit appeals) adjuticators don't try to get fancy or sophisticated with you in laywer talk. They will interrogate you about all your arrests in simple english and in the case of a narcotics arrest, expunging, immunity from prosecution or any kind of plea bargain they don't JUST care about official court records and dispositions...they also care about unsealing sealed records. Being a branch of DHS they may very well have access to more information than one might think they would just through pubic records of the court system.
Applying for immigration benefits in some ways seems to be a procedure closer to seeking top secret clearance than going through a criminal court proceeding. Anyway the problem that you seem to have missed is that if the AG "has reason to believe" that you may in the past have been addicted, or may have in the past trafficed then that can render you ineligible to ever recieve immigration benefits regardless of expunged court records. That's the problem. It's complex so you need a complex approach to it too and you need to think outside the box and/or hire a lawyer capable of thinking outside the box rather than someone who just knows how to fill out forms which even a paralegal can do.