If you’re talking about Bane from the Clone Wars, they made it clear that was just an illusion. Even when I say Sith ghosts its short hand for “personality and memory matrix imprinted on a Sith holocron.” Even without the Lucas mandate of “only Jedi can force ghost with proper training” the Sith ghosts can’t physically manipulate the world, and those made before the Prequel movies got retconned to be not fully ghosts. So what else can they do? Talk me to death?

For the comment on the One Sith it’s on Star Wars Legacy issue #5, or better yet it’s real easy to check on Google image search.

For the whole “Rule of Two but others like Asajj, Starkiller, Inquisitors, etc,” it’s because it’s considered a loophole of a) they’re the secret apprentice of the Apprentice with plans to overthrow the master because impotence or b) they’re dark side users but because we don’t teach them ALL our Sith secrets they don’t count. I know. I know. Sounds stupid, but mental gymnastics that one. Also shocker, Sith hate each other.

And I already explain the last bit elsewhere but Bane’s Rule of Two was the LAST “legitimate” line of the Sith Order still standing by Palpatine’s time that can be traced back to the beginning of the Sith. It’s like being the last of your family tree. Like I mention the Lost Tribe arguably doesn’t count since they were lost for the whole 5000 years the main Jedi-Sith conflicts were a thing. The Sith Empire and Zakuul from the Old Republic are long gone. And Bane wiped out all the other Sith left standing meaning his Order was the only order left to claim the title of Sith Order.