She is an Antique Weapon and a giant Mermaid, if she is trained to become a fighter she could be strong not to mention the fact that she can control Sea Kings.

The technique consist in manipulating movement and strength so as long as the speed remains slow enough she cannot interact with it because of her own lack of strength. You could see it as Rogue bear hugging her when she is at the ring extremity, with his arms blocking both sides and being unable to advance or retreat .

My reasons for picking Kureha above Law is that her medical experience is far beyond his, the MC will use the Ope Ope no mi to become immortal with his crew and Law is either in North Blue or in Grand Line which makes him difficult to find before he becomes a pirate. As for Ivankof Horu Horu no mi, it doesn't only have the miracle hormone who heal by draining the life expectancy. The other effects include boosting strength, reactivity, growth and others depending on which hormones are used.

They are still going to age at a normal rate until they reach adulthood. When they have their adult bodies Jewelry Bonney will just manage their age to keep them during their physical peak.