I'm not sure which gas is used on manufacture, but I believe that there's a vacuum inside the finished fan blades. There's a piece of GSE called a Tap Tester which is used in checking for cracks in the blade, and it works on the principal (IIRC) that any cracks would allow some air to seep in, affecting the acoustic properties of the blade.
On the first question...I believe the issue is weight. As engine diameters go up, the associated weight of a full length cowl increases. At some point, the benefits from a noise / efficiency viewpoint are outweighed by the increased engine weight / drag. It gets evaluated seperately on each new aircraft + engine type.
Hope this helps, but you could drop a line to RR for a definitive answer.