The characteristics of Cthulhu and his “people” seem to be the same, except Cthulhu is larger and more powerful-presumably this is why he is their leader. As for “High Priest,” what are the details of the religion followed by Cthulhu and his kind? Do they in fact worship deities? Cthulhu is said to wield powerful magic, sometimes given as the reason for the immortality of his race. Is this what is meant by “priest?”
The Outer Gods are depicted as mightier than the Great Old Ones, though this distinction seems somewhat moot, as the Great Old Ones live eternally, cannot be effectively harmed and are possessed of virtually limitless power. The Outer Gods, with the exception of Nyarlathotep, are largely depicted as mindless and essentially the embodiments of cosmological concepts. Perhaps this flips the power differential between them and the Great Old Ones.
Perhaps Cthulhu is a great scholar among his kind, and this, combined with the protective magic that ensures his people’s immortality, is what qualifies him for the appellation “priest.”
Some people view R’lyeh as a prison in which Cthulhu and his kind are held. However, it should be noted it was their city and dwelling place until the stars “became wrong,” at which point it sank into the ocean depths, and these beings fell into what appears to be the equivalent of a coma.
Lovecraft, in the character of Castro, said this about the Great Old Ones: “When the stars were right, They could plunge from world to world through the sky; but when the stars were wrong, They could not live. But although They no longer lived, They would never really die . . .” — H. P. Lovecraft, “The Call of Cthulhu” (1928)
Also in “The Call of Cthulhu,” Lovecraft wrote “The nightmare corpse-city of R’lyeh was built in measureless eons behind history by the vast, loathsome shapes that seeped down from the dark stars. There lay great Cthulhu and his hordes, hidden in green slimy vaults.”
Personally, I love the visual of Cthulhu and his kin “seeping” down from the dark stars.
Cthulhu’s race are said, specifically, to have traveled to Saturn–though from where we are not told–then to Earth.
Thus, we know the Great Olds Ones are generally able to essentially fling themselves at planets as they wish–an odd image of space travel–though that may not be precisely how it works. Many things are couched in simile, after all.
R’lyeh, then, is only a prison in the sense it is where Cthulhu and his people are currently, while they are in some sort of deep slumber. Presumably, the other Great Old Ones are in the same sort of suspended animation, scattered about the universe.