> My question really is whether they could not be seen as > <comb>a<comb><comb>diaeresis</comb>, etc. Where the shape of > <comb>diaeresis</comb> is contextual.
Sorry I did not understood the question. Anyway I don't see the exact problem you may find in this case. There are other stacked diacritics in this document (such as breve plus acute accent), and there's probably no need to encode them in a precombined form, and which problem it would solve. The good question will be to find fonts or renderers that will correctly render those unusual stacks (of a combining letter plus a combining diacritic, over another base letter).
