Neither the Guardian nor Greenwald are "read in" to the U.S. national security information apparatus. They literally have
no legal requirement to avoid publishing what they have.
As far as being Disappeared, even Greenwald can't possibly be that dumb, can he? If he has information that the NSA would want to remove him for, and he hasn't already deleted it, it is by far in NSA's best interests to "disappear him" right now.
If they wouldn't find it politically possible to disappear him now then it wouldn't be politically possible to disappear him then.
Does he just think he's going to string NSA along and that they're simultaneously angry enough to kill him and dumb enough to let him choose to leak it after all?
The very fact that you (and surely many others) feel that Greenwald might release this "super sensitive material" later should be all the reasoning you need to understand why NSA will have to remove him now or not remove him at all.
Obviously I'm saying the latter will happen, but this means that Greenwald is acting like a tinfoil-hat conspiracy nutter. Hell, even if the former will happen Greenwald is acting like a nutter; his logic is all wrong even for the world he thinks he lives in.
Once he releases the information he can pose no further risk to NSA for now, which would make harming him far more politically damaging (as it would be revenge, and not to protect further disclosure).
I don't think he's stringing NSA along at all, I think he's stringing the rest of us along.