I wanted to reply to my own questions (let's say 1-4) with some follow-up, because I didn't realize how complex the system was until I read the linked article.

1) The article mentions the drop locations need to be places where another person stumbling upon them will be super unlikely, and which also haven't been used before (not regular, recurring locations as I assumed).

3) There is still likely a fair bit of risk to the buyer, but the writer seems to believe they can plead innocence

4) Addressed in the same as 1, and with drop locations being new every time, making more involved drops may not be as much of an issue

(2) still begs the question, people making lots of drops seem to have a lot of risk; if many controlled buys are taking place, officials can crossreference the lists of people who have been near that location. Once they identify people who make the drops they can then watch the people who are seen in many of the locations, subpoena location information from Google, etc. until they catch them in the act of making a drop or gather enough evidence to indite them. These people could then face very serious charges of conspiracy. It's not unlikely that these people could then be turned informant. Get enough surveillance on, or cooperation of the people making the drops and you can then use the same techniques to suss out the sales layer. Rinse and repeat.