I'd say yes, it was fully seated. The weight of the balls above the carousel kept it fully seated.
That said, when the hopper popped off the back "anchor", somehow a ball got underneath the carousel, say you could easily imagine that the carousel must have lifted up a little bit to allow a ball to get under there.
The shaft that connects the carousel to the gearbox is keyed with a long flat, matching the shape molded into the carousel. So a slight lifting of the carousel wouldn't in itself prevent the carousel from turning.
Thanks for the suggestion, but I think it's more serious than a seating problem.....though, until I know for sure, I'll hope that it's that simple.