Yes…that rock gave the island its magical properties…
Probably because he wasn’t a part of the group.
Nikki and Paulo were not the main characters. Eko didnt want to come back to the show. He hated living in Hawaii…not sure about the Stewardess…she wasnt the main character anyways…
Iliana was send by Jacob to bring Sayid back while Ben brought the rest because they left prematurely before they could complete their mission. Iliana also brought back Locke’s body to show Ben that he was taking orders from MIB impersonating as Locke.