Currently I'm of the belief that Im has been alive this entire time and might even be Uranus; that Im is what the Elders are so afraid of.
My current thoughts are:
The Celestial Dragons' ancestors won a great war but that victory meant sacrificing the will of the world to Im.
They might not all have been AHole families, but they saw the futility in going against Im and submitted….except for Nefertaris.
This irked Im because Im was infatuated with (who I'm going to call) Ancient Vivi. We know that mother-daughter share a strong resemblance. It could be that Vivi bears a strong resemblance to an ancestor that Im wanted for itself (to love? to kill?) but was robbed of when they left.
The CD now act as avatars of Im. They cover Im's existence and are themselves considered the highest being when in fact its Im who is working mostly everything.
Why is Im hiding its existence? Perhaps because it fears the return of whatever threatened its power/influence long ago.
I'm a tad suspicious on this part of your explanation. Maybe it's just me but adding love, infatuation, obsession into this feels slightly off to me. I do find it possible as a reason but also slightly…not? Maybe because Vivi looks like her mother, who probably looks like her grandmother and so on. If his infatuation was that central to him killing Vivi here then there were plenty other look-alike beforehand to eliminate.
Of course, this can be explained by Cobra off-handedly stating that all previous Nefertatri females were mysteriously offed. At the same time, such an infatuation could be the reason she was chosen as the light to be erased (if she really is the chosen one) because I cannot see a reason why she would be chosen and not Cobra if it was solely about Alabasta.