oh I have THOUGHTS on this :3
About Ranni's involvement
we know two things. First, she was 100% involved in the plot, even she's probably lying about her role. Second, we do know what her ultimate goal is : to sacrifice everything and take down the divine rule of the GW/Elden RIng/Golden Order over mortals with her.
This self sacrificial attitude makes her very similar to Miquella, but they go about it in opposite ways. While Miquella' stance is one of Martyrdom,sacrificing himself to divine power in order to give its "blessing" to everyone, Ranni is sacrificing herself to make sure no one has ever to suffer from the divine power's blessings, that she consider, undderstandably, more of a curse than anything else.
This also manifest in their relationship to their follower. Miquella used his charm freely, and doing so amassed a devoted mass of (sometimes fanatical) followers. Ranni, as a narrative foil to Miquella, takes the opposite stance. Has she knows her road is one that lead only to misery, she do everything to walk it alone, only accepting the help of her most trusted followers. Most of her personnality is based on rejecting bonds with others, as she's constantly distancng herself from everyone and even define the relationship of her followers as "taking what's they need from the others". There's a big rejection of love as a concept here, because she know (maybe from observing Miquella) how much damage love for an empyrean can cause.
But, and this is extremely important, she do love others. It's one of her core traits, and I think the main reason behind why she acts so distant : she cars about those close to her so she doesn't want them to get involved in a goal of sacrifice. By bearing her pain alone she avoids spreading it to others.
This understanding of her character makes her attitude related to the Plot quite easy to understand. While she did take part in the plot, and we know she had a good reason to, she lies to you when asked, saying "I did it all" to take the entire blame for herself, despite the plot being a collective effort between at least 3 different factions. This is again her self sacrificial nature showing, bearing all the weight on herself to save others from it. (And remember, when she explains this to you you're pretty much a stranger. Why would she be sincere ?)
About the Black Knives Assassins
The situation here is extremely messy. But we do know a few things for sure about them : They were from the Eternal City (cities?), they were related to Marika, and Ranni gave them the murder weapon. This leads to a bunch of question, and a lot of speculation that I will try to answer.
Why did they do it ? We know the eternal city attempted to create their own lord of the night, and that it failed terribly, and the the greater will banished them underground. With this alone they'd have a pretty good reason to recent the golden order to orchestrate an assassination. But. We also know that Marika is related to them. So while they where banished by the Greater Will, at least one of them got on it's good side. And I won't say I know what Marika did, but I could definitely see the Black Knives wanting to take back what they consider rightfully theirs. murdering Godwyn in this case makes perfect sense, as he was the incarnation of the gift of Gold.
Why did they cooperate with Ranni ? they at least have some common ground : they both want to take down the golden order, they both want the coming of an "Age of Night", and they both want Godwyn dead. Ranni's desire for Godwyn's death is a bit complicated : either she just targetted him because he needed to die for a new age to happen, or she needed him to die because she refused an implied political marriage (remember that there is no official family tie between the carians and the golden lineage, so a mariage between the two heir of the royal families would be a perfect excuse to get back the Carian kid as ruling demigods in Leyndell. Also the ecclipse is the union between sun and moon). Either way, they both have everything to win by working with each others.
Why did the Black Knives turn against Ranni, then ? I guess because Ranni double-crossed them. I doubt the assassin ever knew about her desire to also kill half of herself, and I doubt the Black Knives "Age of Night" would have involved yeeting divine influence into space. And since Godwyn didn't fully die, the Golden lineage couldn't be erased for the new age to begin. So while the plot was a full success for Ranni, it was total failure for the knives. No wonder Ranni is hiding she has a lot of enemies.
Why are there so many Black Knives in Ordina ? I have no idea if Miquella was involved in the plot. But what is there in Ordina ? Albinaurics. And it's not just an average albinauric location because the hope of their entire race is nearby, as Latenna big sister who is expected to accept the drop of dew (or something like that) and allow a new generation of albinaurics to come. The presence of both albinaurics and Black knives helps us tie a lot of lose threads together.
The albinaurics are made of a silver metal, they are all born of a (same?) mother despite being artificial, and their creation is often represented as the ripples of a drop of dew. Silver metal, artificial life, prolific mother and drop of dew are all important themes related to the Eternal City (and the Carians). So, it is quite reasonable to guess that the albinaurics and the Black Knives had some common goal.
What's the deal with Marika in all this ? I have no idea. There's a common idea that she participated in the plot, but I really can't see a good reason for why she would want to murder her only non-cursed son, that she worked so hard to conceive.
And what's her relationship to the black knives ? I also have no idea what happened, or what she did to them for them to get so revengeful. But there's probably some juicy family drama here. We also know that Marika commited a "seduction, and a betrayal" to create a new Age so this may be related ? But it's impossible to formulate an hypothesis without getting into wild speculation here, there's just to much unknown.