His skin could’ve been augmented by Vegapunks experiments. Vega could also removed the scales whenever he went in for upgrades. He would still be sick regardless of the scales advancing or not.
Saturn gave the order, he agreed because he was dying anyways and maybe made a deal with the WG to leave Bonney alone or protect her. We have seen several WG characters worry about Bonney
He spent 2 years guarding sunny with no sign of turning blue though lol, then as a slave in Marijoa. We are assuming a lot of things here:
- That he can take diseases with him
- That he hadnt tried to take the disease from her already
- That modifying his body stops the disease.
If turning him into a cyborg stops the disease, then it doesnt explain why he "needed" to get his mind wiped, especially since hes still alive with no signs of a disease affecting him