The NLA is more of a 'global' animation system in that you can track and animate objects and items, it's basically to top level system where objects and events are animated rather than bones and armatures - you can animate characters using it but it's messy as it's not easy to drill down into the working data. For example you could rig a 'walk' anim for a character, armature legs and body move, but no actual forward motion. Using the NLA would then allow you to move the character forward in *object* mode - you'd *never* do this for an actual game character though, I just use it as an example of how the NLA can be used.
Because of the way I rig my characters, I've never yet had a need to use anything other than the Action Editor, messing with IPO data and what have you has never seemed that intuitive to me.
So in answer to 1) yes (I've got some tutorials in mind to explain why but finding the time as always). 2) As far as I understand, yes, Blender works and relies on those Datablocks, so you're correct.. if there are no connections to them they tend to just sit there - you have to re-load and save the file a couple of times to get rid of them. And again yes, if there are no connections to the datablock they can't be exported from what I understand.
And I agree, Outliner for the win (certainly the way it used to list everything), I often find myself referring to that to see what's in the file and connected to what.. especially the OPPS Schematic of pre 2.49.