IDK if it's too late to discuss this, but a third possibility could have been to use the second character or more of a # comment as a marker for diffscuss.
I like this idea. And how I see it this isn't really mixing comments and functional stuff, since the code reviews and replies are really just comments on the diff, which is the intended purpose of the # comment
But the diffscuss modes parse the comments into functional structures--threads--to decide where to insert replies, etc., which is in fact functional--just not functional for the diff part.
Here's another way to think about it:
It sounds exciting to think that "all legal diffscuss files are also legal diff files!" In fact, very few diffscuss files will be used as diff files, and those that are can be trivially transformed.
It's not awesome on the other hand if "not all legal diff files are legal diffscuss files," which is where piggybacking on the diff comment syntax gets you, because every diffscuss file starts its life as a pure diff file and there's no way to transform an offensive (but legal) diff file into a legal diffscuss file without data loss.