1. You should consider “c’en est/était/sera/serait… fini de sth” as a single unit, a set phrase. En has no explicit antécédent here like with the expressions en finir avec sth, s’en aller…

2. You can’t. C’est fini des pigeons voyageurs is not grammatical although you might write:

   C’est fini, les pigeons voyageurs.

   or better:

   Les pigeons voyageurs, c’est fini.

An added thought to the above… If you compare “c’en est fini” to “c’est fini” the first one is basically indicating an entity (the practice of using carrier pigeons) that is being ended. It can be used to add more emphasis to the whole of what is being considered finished/over in a sentence than just “c’est fini” (tho “c’est fini” with an exclamation point can certainly do that too). The “en” is talking about the end of the whole practice of using carrier pigeons.