1) No, and never will be. HK DOES NOT SELL RIFLES TO CIVILLIANS ANYMORE.
2) See #1. IF they are adopted (and fortunately, that's a BIG IF) the only way for you to get your hands on one will involve a visit with your local recruiter, at least one 5AM visit to MEPS, and 9 weeks of being chased around by a bunch of loud folks in smokey-bear hats...
3) See #s 1 & 2.
However, some here have fired them. Reviews range from neutral to negative.
One of the guys from CavArms has handled the G36, XM8, and obviously M4s & ARs (as he makes them for a living)... He's also talked to Army Marksmanship Unit folks, and others who have also handled the 'new' rifle, and has NOTHING good to say about it...
Neither do I... It doesn't do ONE thing better than the AR of any significant value, the only real 'advantage' is that it takes less time for a civillian to clean (I say civillian, as the military cleaning standards have very little to do with actual rifle function)...
Further, in an actual comparison, the XM8 'marksman' version failed to produce a smaller group at 100 yards than the AR-derived SPR was able to achieve at 300yds. It is, as expected for a gas-piston driven weapon, an innacurate POS...