Also I'm not sure that Moffat is 'gambling everything on the big River Song reveal' - that's not how it works.

sensiblecat is talking about the potential TV ratings, not the story. S6 starts April 23, the latest start date compared to previous seasons. A full 13-week S6 would run to July 17. The short season with 7 episodes will probably run to June 5, which should avoid the mid-season ratings drop Who's always had. Also, moving the last half of S6 to the fall capitalizes on the larger number of TV viewers compared to spring. No matter what Moffat says, it is at least partially ratings-driven, considering that S5 dropped steadily in overnight ratings over its run. (TPO/TBB had the lowest overnights for any season finale.)