Is this appropriate as a 7 kyu kata?

Please, do another one! This was awesome!

I too believe that 5 kyu is too high - the algorithm is trivial and implementation straightforward.

Thanks... I would probably cringe if I saw that swap in production code :)

The above use of the underscore stems from my experience in statically typed languages that require a parameter, which does indeed mean that it will not be used. One could make an argument that callbacks to built-in array functions (map, filter, reduce, etc) are clearer with underscores for unused parameters. That being said, if I were to write it again in JS, I would probably omit them.