Assert.AreEqual("ROT13 example.", Kata.Rot13("EBG13 rknzcyr."));

Not an M or an @ in sight. Also, M should transform to Z, not to @.

Not a kata issue.

You need to count also the ones that aren't repeated, 15/6 = 2.5. See the last example in the instructions.

The index is calculated based on how many vowels are repeated more than once in a row and dividing them by the total number of vowels a petty enemy of America would use.

This isn't a kata suggestion, it's a question. If you order the numbers in increasing order, and you get the first 11, there is only a single 2.