Ethan

It didn't grow back. It was re-attached with help from the healing items. They have the same, albeit to a lesser extent affect that the mold does. That's why when, Ethan gets his leg cut off, Jack gives him the first aid med and it re-attaches itself.



~Stop Now Before I Kill You All.A Word To The Wise From Your Friend Pennywise~

"If his hand actually grew back and didn't need to be reattached, then why the staples in his wrist?"

Because he wasn't infected, not at that point. It wasn't until after the hand was reattached that he ended up becoming infected in the first place. The whole scenario was a weapons test.