Good news: All that dry-practicing doublefeed clearance paid off. I can clear a spontaneous type 3 like a motherfucker.
Bad news: See above.
(All kidding aside, I discovered a good use for those molded-plastic dummy rounds. The rims break a bit too fast, but when they do break the sumbitches won't come out of the chamber without some gentle caressing. So my "how do I spontaneously induce a type 3 to practice clearing them?" question appears to have been answered.)