Out of curiosity, was the structure of the tree created based on letter frequencies similar to trees within a Huffman Encoder? If so, shouldn't O, I, and S be higher up the tree?
Things like Q get used a lot in Morse for example (look up Q-Codes) compared to real English.
So, traditional letter frequencies are not applicable (Also because the code itself is language-agnostic)
Using a tree like this to decodee also sucks from a speed perspective. You need to learn the sounds /not/ count the dits/dahs if you want to get a good word per minute.
source: radio ham who knows morse and can do 20+ wpm.
A lot of weird stuff in morse and things also comes from older comms methods like Semaphore - The reason we have M in Scottish callsigns for ham radio is because M is the saltire (Scottish flag) symbol in Sempahore, for example.