Island

Island
 

https://www.youtube.com/watch?v=eYjPdLjakO0 At 4:31,

-I will arrive in Melos Island momentarily.

https://www.youtube.com/watch?v=fV8Qt2wlcEI At 0:01,

-The Seno League at LeeLee of Valy Island is heading into the final stretch.

American voice actors speak very fast; but assuming I hear the correct thing, what's the difference between in and at an island?

It's the verb what determines "in" in your first sentence.
- I will arrive in London.
- I will arrive in Melos Island.

The "at" is defined by the place (LeeLee) of the island, not the island itself,in your second sentence.

This is wrong. Even when arrive is used, whether we use in/at depends on the location. I hope a native speaker will provide a little assistance.

I prefer 'at Melos Island' when speaking about a destination such as an island, but 'in Melos Island' is also correct. And, if you're not arriving, 'on Melos Island' is also correct.

See if this video helps any.

Excellent answer. Thank you.