She could just not get sick or poisoned? This is fiction. Oda can just write it so nothing bad happens. She can just bring food for the trip or not eat like she did in Wano, and no storms happen to bother her, she just follows the log pose to egghead. It's really not complicated. You're just trying to invent problems.
One Piece is fiction from Oda. Yamato sailing out to sea alone either in a ship or skating across on the sea is fiction not in One Piece.
You are stating Oda can just draw and write Yamato however he wants. This is true for the most part. But my argument would be that Oda is meticulous. Yamato and Momo are both 28 year old Mythical Zoan types that are Guardians of Wano...whose fathers fought each other...Momo and Kaido are dragons...Yamato idolized Oden...these connections Oda brainstormed together. To likely stay together. Momo is not surpassing Yamato anytime soon so Yamato will not be possibly leaving until then.
Yamato has shown no inclination of knowing how to sail. Only prior knowledge is likely from Oden's Journal. Unless pre-Wano Yamato was out traveling the seas with Kaido and mother. Oden could not leave Wano alone, needed Whitebeard Pirates. Yamato was going to go with Ace. Then Luffy. Perhaps it's the next crew to visit Wano...or leave with Captain Carrot.
How long of a journey is Wano to Elbaf...by foot? Also why would Yamato choose Elbaf first? Yamato is free now yet chose Wano. Wano is Yamato's new family. I would guess Yamato sails with Momonosuke, Retainers, Tama, etc... if Yamato goes out to sea.