Dodged the bullet there!
UKNATIVE:
PC 1000 is a very good resolution of a case like yours; especially considering that original charge at the time of arrest was a Felony, later reduced to a Misdemeanor and the fact that you did not have anything similar in your past made you eligible for it. You also had the advantage of not being a 4th Waiver during the 22 months period (4 months of PC1000 and 18 months of summary probation) and on to the final stage, which is the erasing of all traces of the incident from your permanent records, including the arrest.
That, however, is good only in instances of job interviews, renting a property, etc., on which forms you can answer NO to questions of whether you have a felony conviction or if you were even arrested for it. But, criminal records will still be visible to law enforcement and all other government agencies, including good ol' I.C.E. of the immigration fame.
So, do not even attempt to answer the question of any arrest or conviction in the negative; just say it as it happened and at this time you should not worry about this incident affecting renewal of Green Card as it was only a misdemeanor, and the fact that you were not convicted of it; all you did was plead guilty to the charge and opted for PC1000, which made you eligible to have ADJUDICATION WITHHELD pending completion of the program.
The only time you get in hot water with I.C.E. (no pun intended) is if you accumulate 2 or more misdemeanor drug charges that have actually resulted in convictions. This will then elevate your status with I.C.E. to that of an AGGREGATED FELON and so subject to immediate arrest and removal from the country. So there is no need to worry at this stage even if you were convicted of the misdemeanor drug charge since it is, has been, and hopefully will be the only blemish on your record.
Now, none of the above will have any relevance or affect whatsoever, to you or your status if you are already a Naturalized U.S. Citizen because you can only be removed for criminal indiscretions if you are NOT a U.S. Citizen.
By the way, full marks to your attorney for advising you to plead guilty in return for PC1000.
fredrikklaw