This is the whole point of marking them up as one or the other. This goes against the very foundation of your discussion thus far saying that they need to be allowed as a child of one or the other.
Either users need to know this therefore the markup needs changing or they don't need to know this and what we have currently is just fine.
Unless you provide a case illustrating your point beyond a doubt that change is necessary, I'm not sure what anyone can do here to influence vendors into changing. You're starting to grind against your own path it seems.
The order should be read in the order they are listed in the document. Is there a known case where this isn't done?
Not true. You only can't define them as ordered via markup they are inherently unordered.