1. If the LMS had purchased the Castles, They probably would've had a similar destiny as the Royal Scots did. Even though he came from the GWR, Stanier would probably modernize the Castle design, but maybe keep the four cylinders. Not too sure about the Patriots and Black 5s. The former would probably just be Castles with three cylinders instead of four, and a slightly smaller boiler.
2. Don't think making the Kings a Pacific would've done them much good. Their route availability is slim enough as it is, and building them as Pacifics would probably put them in the same position as The Great Bear. Actually, didn't Hawksworth propose a Pacific? The Cathedrals maybe? Or the Queens?