I need answers...NOW

Sounds like your just bad at video games.

You can put a carefully made boot around a finned tail and it'll stay on because the fins help it stay in place, but putting one around a snake tail means it either slips off or has to be tied tightly enough that it cuts off circulation. It's simply physics!

Well they are actually hats for the both of them, thus they must be worn on their heads, Nami can place them on any of her spiked up head pieces while Cassio has no where to put it, they would just fall off.

Scientifically speaking, these depictions are highly accurate for a woman that is half snake and a mermaid.