Yamato's character arc is over.
The point of creating Yamato and Momo like two sides of a same coin (because you are right, they are) is like I was trying to explain in the last Nakama thread : the way Oda used to make their thematic have more echo. The thematics of the inherited will is phisolophically debated throught both Yamato and Momo's characters (and also a myriad of other characters) but also the thematic of Parent/child relationship. As both children will eventually go into the opposite direction of their biological dad.
The theory of Pluton is fun to theorize but it has not storytelling evidences behind it.
The reason why Yamato is the way he is, is because he is portrayed as the opposite of his dad. Fire > =/= < Ice. Right now, this is the simplest and most efficient explanation for this power. He is also designed as the gardian deity but most of all, Yamato was not only the name of the warship.. it was the name of JAPAN. Which in One Piece.. is represented by Wano.
Yamato is simply depicted as the guardian deity of Wano. Nothing more. He might come back in the story, but his character development - at least in term of lore - is over.
As for Lucci. He was depicted as a clear murderer and a fascist enabler. He won't join the strawhat, there is no place for a redemption arc for him in the context of the way One Piece creates redemption arcs.
