I'm sure you're more knowledgeable about this than I am, and I'm sure I'm showing my ignorance, but isn't it possible to support both at the same time? I was under the impression that, for example, c-cedilla was one character and "c" plus a composing cedilla was two characters, and that the two constructs are completely disjoint.

I guess my question can be summarized as "is a string containing both c-cedilla and a "c" plus composing cedilla couplet invalid?". That is, it seems to me that "<U+00E7><U+0063><U+0327>" should be valid and be rendered as "çç".