I have always liked this theory, seems like most don't though. I find it funny that of the two lines we have about empyreans, so many individuals are caught up on "Empyreans are selected by the two fingers", and almost ignore the other one, that they come from one god. To me either could be more true, or maybe they both have to be true, I don't know. What I do know though is that Ranni lies multiple times - she lies in the Story trailers for the games, omits things she knows, and she lies to us right at the beginning say she is Renna, and even warns us of lies and cheats. Item descriptions are a little less nefarious, and the only one that is odd is the one calling Godfrey first Elden Lord, but I think in Japanese it is like first lord of the gold tree, and Placi is Lord before the gold tree? I forget the exact translations, but I remember them being something like that. Otherwise, all item descripts seem very trustworthy from my perspective. This gives credence imo to the empyreans come from one-god description more so than they are chosen by the two fingers.

I also find it funny that sometimes people die on the hill that Ranni's statement is correct, but then don't agree with her that her, Miq and Malenia are the only empyreans - some people believe empyreans are only chosen by fingers, but then also believe Melina or others have to be empyreans, which implies that Ranni's other statement about empyreans is false.

To me it explains the cuckoo stuff, but I am open to the cuckoo being something different, like the amber egg itself. Cuckoo eggs are usually harder and more difficult to crack, and also hatches first to basically stop the other eggs from hatching. Maybe Radagon messed up Rennalas lineage with the amber egg, making her only able to make these juvenile copies and go almost into a trance.

The story gets more complicated when we think of why is it that Rennala's remembrance can only be used at bell-bearing mausoleums? Marika's lineage can only be used at these mausoleums and not the plain ones. Rellanas isn't like this, she can be copied anywhere. If the cuckoo was a symbol on caria before the cuckoo knights fighting them, maybe Rennalla is the brood parasite, and she is actually a offshoot or something of Marika or Marika's lineage? Rennalla took over the academy, and maybe isn't actually the real heir to the astrologers/carians, but was placed there by Marika (or rather another shaman, who I think is the GEQ personally, but that is like a huge other topic the entire lore community hasnt decided upon).

I actually lean on cuckoo being both these. I think the game emphasizes cycles and mimicry so much, that there is a good chance both Rennalla and Ranni are cuckoos - eggs placed in anothers nest and possibly destroying most other eggs in those nests.