I think I mostly understand Cal's timeline now, but there's still one thing that doesn't really make sense. Dirk's Cal is empty, and there's a nice traceable path from that to the Masterpiece where it's filled. But isn't Bro's Cal already full which is why he was such an abusive shit? How is the difference possible when Dirk and Bro both arrive with the same Cal after the scratch splits the timeline?

See- The thing is that, while it’s highly likely Bro did hear Cal’s whispers and was mildly influenced by Caliborn, Bro being a piece of shit is inherent to Dirk. His main struggle is repressing his controlling tendencies and knowing the fact that, if left unchecked, he could’ve become a really huge asshole, but that his love for his friends and his attempts to better himself are constant. Just like how Mom fell in to alcoholism, just like how Grandpa isolated himself, Bro represents what Dirk had to overcome and not be like, and putting the blame entirely on Lil Cal, SPECIALLY knowing Bro wears shades and shades are the universal symbol of blocking out mind-controlling whispers, would be refusing to see the flaws Dirk sees on himself and how they affect him.