Cart before the horse?
Just a thought, I think that the original poster stated that the "premature" slide stop problem manifested itself before the last case jams started happening. However, it is my understanding that having the slide stop locking the slide back when there are still rounds in the magazine is usually caused by the nose of the bullets contacting the slide stop. That makes me suspect that, at some point when he was shooting, he may have had a case jammed into the magazine feed lips and bent them allowing the bullets in the magazine to nose-up and contact the slide stop and causing the perceived problem with premature lock-back. In short, that premature lock-back may have been a red herring whereas the real problem may have been the extractor all along. Just something to consider.
Just a thought, I think that the original poster stated that the "premature" slide stop problem manifested itself before the last case jams started happening. However, it is my understanding that having the slide stop locking the slide back when there are still rounds in the magazine is usually caused by the nose of the bullets contacting the slide stop. That makes me suspect that, at some point when he was shooting, he may have had a case jammed into the magazine feed lips and bent them allowing the bullets in the magazine to nose-up and contact the slide stop and causing the perceived problem with premature lock-back. In short, that premature lock-back may have been a red herring whereas the real problem may have been the extractor all along. Just something to consider.