I thought Carrot would join before Wano. She hasn't had the presence I would expect from a prospective nakama. But the situations aren't the same.
The Ace backstory is backstory. It's part of Yamato's history and narrative. Oda didn't have to add it. There was nothing that required him to give Yamato such a backstory with Ace.
It has literally no bearing on her interactions with Momonosuke. Momonosuke didn't need to know about Yamato and Ace's relationship. That scene only existed for us readers to see what Ace meant to Yamato. It was of no use to Momonosuke himself.
Like Yamato's interactions with Momonosuke were:
- Scaring him away by claiming to be Oden
- Being a meat shield to protect him from Sasaki's Armoured Division
- Declaring that her name was Yamato and that she would die for him
- Declaring that he must survive to guide the world to its Dawn
- Fantasising about Ace
- This was basically all they did together when they were not running from enemies. The one time Yamato actually sat down to talk with Momonosuke it was so that Momonosuke could be a vehicle to deliver the Ace flashback.
- Handing him Oden's journal
- Asking about Luffy
- Abandoning him to assist Luffy
#1 was bad comedy.
#2 and #3 helped flesh out and develop Yamato's character and to hype her combat ability.
#4 and #6fleshed out Momo and explains his importance.
#5 was 14+ pages and it was the majority of dialogue that was shared between Yamato and Momonosuke. No explanation of why she would die for him.
- Not an explanation of the Dawn of the World or why he was so important to it.
- Not an explanation of some of the details of the journal.
- Nope, Yamato fantasising about Ace, and how it's actually Ace's brother Luffy and not Ace that's the child of prophecy.
#7 & #8 was Yamato moving on to more important business now that Momonosuke had outlived his role in her narrative.
If Momonosuke was really as important to Yamato's narrative as you're claiming he wouldn't have been reduced to a vehicle for Yamato to fantasize about Ace and Luffy.
If Yamato's purpose is to truly become Momonosuke's guardian, she wouldn't have spent her time with him fantasising about Ace and Luffy.
If Yamato's purpose was so tied up with Momonosuke's she wouldn't have abandoned him for the chance to help Luffy vs Kaido.