Does Unicode expect a user to accept having Unicode ordering forced on them when it makes no sense and is not grounded on how the script works?
The correct approach, as present in the Unicode standard for many years, is to render canonically equivalent orderings the same way. Once you do that, you will handle both normalized form, and whatever equivalent form users input, without worrying (or having to precisely establish) what the ‘correct’ order is. This is not a huge burden. The amount of time necessary to reorder combining marks is completely immaterial compared to the time required for other work that needs to be done in rendering. And notice that the rendering engine could reorder the marks internally in a different order if it wanted to, as long as that order was canonically equivalent. In particular, any permutation of the non-zero CCC values can be used for a canonically equivalent internal ordering. So internally a rendering engine could permute weights <27, 28,..., 32, 33> to <33, 27, 28,..., 32>, getting SHADDA before all vowel signs, for example.
