Sweet, I think it needs this to stay comparable with other models from Lexus and others.   

What are the advantages of 7sp auto over 5sp?

Their logic is rather than going with a very tall overdrive, or double overdrive, similar to what you see with the T56 on a Corvette, more narrowly spaced gears so that you always have the engine in its optimum power range, or geared for optimal fuel efficiency at lower speeds.  Apparently fuel efficiency at 80 mph was never on their rader.  It'll help city fuel efficiency with no benefit to highway.

Well the final gear ratio would determine most of what you asked... cruising noise, cruising rpm, crusing mpg.  Infiniti decided to gear the 7 speed in a way in which the final gear in the 7spd isn't much lower than the final gear in the 5 speed.  Therefore, the noise, rpm, and consequently mpg are not varied much. 

As I said in my other post... its somewhat obvious they were going to power when they designed this... keep the car in optimal power band and therefore make it quicker... less gas pedal necessary to accellerate COULD lead to higher mpg's.. much the same way my wife's Jeep 5.7L gets almost identical gas mileage as the 4.7L because it doesnt have to work as hard with the same gearing to accellerate at the same pace.

Its an all new transmission, so I wouldn't think it would "carry-over" unless they put it there on purpose

That's a short ass first gear.

Look at what the current 6 speed MT offers

1st - 3.794
2nd - 2.324
3rd - 1.624
4th - 1.271
5th - 1.000
6th - 0.794
Reverse - 3.446

So,

Since I am car-mechanically-challenged, someone please give me details on how all the gear ratio numbers work out..   I could googol but its more fun this way!