Why do I reimport 1.swf ? even though I have 1.swf into library and it's already on the stage.
You can't add .swf files into the library. When you import a .swf file into the library, Flash creates a movieclip equal in length (based on the current framerate) but only showing the first frame of the imported.

Why use import to stage from directory instead drag 1.swf from library to stage?
Same reason as explained above.

The code animation.loadMovie("1.swf") seems like it runs file 1.swf from directory, not from Library, so why do we need to have 1.swf in the library ?
You don't. That's why I said that technique was more efficient.

If I put just .jpg with static background at the layer1, then rename it to animation, and put that code at the first frame, layer2..Can I? because I knew I have 1.swf in the same directory.
Yup. Thats exactly how you should do it. Any other way is a waste of resources.

Also, like I said, you should probably add a preloader if you're adding this to a website. I linked to a tutorial above. You'll want to watch "External SWF Preloading".