The 7 trumpets being the 7 conventions were originally interpreted by Rutherford as the 7 books released, which all could purchase for the right price. But when I had in my teens read that in the revelation book, at 14 years old, I knew something was fishy there.
I too recall thinking how improbable it was that one of prophecies in Revelation was fulfilled by a single talk/resolution at a single convention in the 1920s. That same question came back into my mind back in 2003 at the DC when they read off that very awkward 10 point resolution:
I still remember sitting at the desk of the Cleaning Department while the thousands in the arena answered back with a verbal "aye", thinking 2 things:
1. The (non-JW) convention center employees who are hearing this must think we're a bat shit crazy cult, and
2. 10 years from now are they going to try to claim that this resolution fulfilled some Bible prophecy?