1) extending Seam components is recommended practice (but I don't understand what you are asking it is better than)
2) All components are equal in Seam (well some have a higher precedence than others), even the ones in Seam core.
3) @Install allows you to run the @Create method on Application and Session scoped components when the application/session starts
4) No different than building Seam components for your app (perhaps we need a bit of docs on this as it is an FAQ).
We have no docs for the plugin stuff yet, no.