So Imu is basically a lunarian with horns and a tail. I wonder if it's related to the fact they were mentioned to have lived on top of the redline all this time before their "extinction".
However the fact he has other traits like horns and tail might suggest other/deeper bloodline for him. Either this or he got these new features because of his Devil Fruit, with the name and tail supposed to match the Devil 😈
But it also basically confirms that Imu himself comes from the Void century and thus doesn't come from the first world, and thus makes it even more possible for him not to actually be the final villain
As for the Admiral/Gorosei/HK debate, I'd say it depends on the wording, but from a logical standpoint, I think it was more referring to Imu sending one of his top forces(admiral/gorosei) to Elbaf, which at the very least further confirms the admirals are above HKs depending on what's going on with Shamrock, the holy land and whoever is attacking it right now. Like for example, if they aimed at Shamrock going back to the holy land to protect it, it could explain why he won't be included