1) Not that I know of. It would be nice if every node were also a viewer node, which would save some time and hassle. As it stands, you’ll need to hook an output node to each output that you want to examine. I usually hook up several and leave them in place while I’m creating the group.

2) The best you can do here is hook an output node to each output you want to compare and click back and forth between them. You can have as many output nodes as you want, and whichever one is selected will show up on the background.

1- rather than keep adding viewers to see output only, connect all to 1 and toggling somehow.
just like in nuke / combustion / fusion >> two outputs and you can see 1
2- also is there a way to compare difference between two outputs >> wipe (virtual slider to move on screen and see difference)
waiting again..

You can use the file output node to save individual passes (Add->Output->File Output). This is what you want I think. Then you can load the separate images into any program you choose for compositing (After Effects, GIMP, Photoshop, etc).

You can even load them back to the Blender compositor with the Image node.

Hope this helps.

for simple scenes / renders, this approach is good, comping before rendering and all modifications will be part of it after final render
issue: if shot / scene is bit and been divided into few layers. comping each layer and then rendering final shot, scares me a bit
like typical approach, i want to render each layer with elements (diffuse,alpha,spec, shadow,reflection etc) so they can be loaded into compositor and have no dependency in scene 3d data. how to setup this.
currently confusions in my mind are
1- when i typically hit “render” key >> blender shows me composite view (where all elements are combined), how can i write each element to disk in jpg / tga / exr??
so they can be loaded in compositer??
btw, thanks again in advance