Phase 1 the bare accent is translated to universal-character-name, so I don't think there is a detectable difference, at least that way, for  #define accent(x) x ## \u0300 vs  #define accent(x) x ## `
Now, in phase 3 we are converting to preprocessor-tokens and whitespace. So we get {x}{white-space}{##}{white-space} and then I think {\u0300}, not {\}{u}{0}{3}{0}{0}, because \u0300 is a "each non-white-space character that cannot be one of the above" , on the theory that if it wasn't a combining character, but instead something valid, like \00C0,  À, we would expect the result to be the two characters pasted together? That is a 'universal-character-name' names a character. 

One of the reasons to eventually get back to my paper trying to clean up 'character' and related terms, because they turn out to be much more complex and vague than expected. It needs substantive revision, though, because some parts that are a bit ambiguous I had interpreted other than how it seems to be intended to be interpreted.