A few questions...

So how exactly do they get away with all the deaths? Sure, knowledge of the island is pretty hush hush but people have heard of it otherwise they wouldn't be able to go there. I'm sure people are going to be pretty hyped and tell their family and friends they're going there. It's going to look quite suspicious when all these people never come back. It will get investigated.

It seems the Island's source of the magic for want of a better word is the stone in the fountain thing which infects the water that runs on it. It didn't seem clear if the water was key to making things work though.

How did Mr. Roarke know if he didn't grant the guest's fantasies, then his fantasy of his wife being there would fall apart?

If Gwen was allowed a second fantasy after knowing how it works, why not Mr. Roarke too? He could wish for his wife to be back without whatever terminal disease she had.

Why did Brax have to stay? His brother was resurrected but could only live if the wisher stayed on the island?