Quick answers:
NSS is based on standard deviations--the whole bell curve thing. 50 marks the middle of the bell curve. This isn't Lake Wobegon--not all the children are above average.
The Osprey pipeline is still up in the air, as is the Osprey program. When they figure that out, it will still be some time before they need boots to start filling it. Last proposal I heard was that one would go all the way through helos, then go to Corpus to fly a modified prop syllabus.
The Corps does not have a special "higher speed" unit, apart from the MEUs, which, being Spec Ops capable, are already "high speed." Marine Recon and the like are transported by regular Marine birds. In Afghanistan we flew guys from every service and some from foreign SpecOps, as well as some guys that we weren't sure where they were from. Some planners began to favor Marine lift over USAF, because we said "yes" when no one else would.
There are limited numbers of exchange slots at TF160 and USAF SpecOps for Marine pilots.
Phrogs phorever