Impossible to decrypt without the key/legend. With the legend the person decrpyting would know which letters, numbers, and symbols didn't belong. The method used for creating my legend is impossible to reverse engineer. Without the introduction of random numbers, letters, or symbols a brute force attack could easily break through. But with the introduction of numbers, letters, or symbols at random intervals I think a brute force attack would be ineffective.
As to the question about the multiple letters with the same encrypted counterpart, I didn't really think that through... I guess at the moment they would have to do a lot of guess and check work to decrypt it even with the legend.