I can answer #1. The issue with #12 as the third hole, was that as the lowest spot on the property, and lack of sun, it tended to be affected by frost the most. By reversing the nines, it allowed tee times to be less affected by frost delay. As far as the effect of the tournament, there was an issue about having the final green (now #9) in an inoppurtune place for galleries. Cliff Roberts was highly cogniaznt of spectator viewing angles, which is why for some time the mounds surrounding 8th green were neutered for many years, to enable viewing of the hole.
As far as the streams turned to ponds, it could have been because of damage caused due to flood surges. #11 was dammed up when it was severely damaged by a flood.
There were many people who had an impact on changes to the course...previous players, Maxwell, George Cobb, Byron Nelson/Joe Finger,RTJ, Nicklaus, Fazio, not sure about the speciifc rounding of bunkers vs the original freeform style.