I fully support this.
The problem with your wording is that you are not allowing Marathon maps to be named "Marathon" anymore (or similiar names), so you have to change it a bit.
Also "Marathon maps with a single difficulty must follow the hardest difficulty rule, and use an appropriate custom difficulty name related to the song as well" is a bit useless since you could just write "Marathon maps must use an appropriate custom difficulty name related to the song as well"
The problem with your wording is that you are not allowing Marathon maps to be named "Marathon" anymore (or similiar names), so you have to change it a bit.
Also "Marathon maps with a single difficulty must follow the hardest difficulty rule, and use an appropriate custom difficulty name related to the song as well" is a bit useless since you could just write "Marathon maps must use an appropriate custom difficulty name related to the song as well"