How do Ashfur's powers work?

I feel like I'm missing something, since English is not my preferred language and Arc 7 has not yet been fully translated into French.

Do we have the answers to the following questions:

1) How does Ashfur become so powerful?

2) Where do his powers come from?

3) Firestar is also able to possess a cat (Rootspring), so Ashfur is not an isolated case... How was he able to defeat all of StarClan?

4) Why does no StarClan warrior have the same abilities?

5) How did Ashfur build another passage between the Dark Forest and StarClan? And by the way, how did he condemn the first one?

6) How can he have so much influence on ghosts?

7) Okay, his connection with Shadowsight is cool. Ok, I like the idea that Shadowsight was never able to communicate with StarClan, and that he was permanently cut off from them at the end of the Arc. But it contradicts everything they've put in place since Shadowsight's birth! Why would Ashfur have sent him a vision to unify the Clans at the end of Arc 6? Or to save the Tribe, in "Tawnypelt's Clan"? It has nothing to do with his plan, nor any interest for him... Clumsy explanation that deserves to be explored further, inconsistency for the plot or misunderstanding on my part?

Thanks in advance to those who will take the time to give me developed answers!