You're correct, the routing mesh does not behave in quite the way described by the docs. We're working on evolving away from the global backlog concept in order to provide better support for different concurrency models, and the docs are no longer accurate. The current behavior is not ideal, but we're on our way to a new model which we'll document fully once it's done.
In the meantime, you shouldn't have any difficulties as long as you keep your web requests short (less than about 500ms), which is good practice anyway.
Sorry for any difficulty or confusion, and thanks for digging in and providing such a detailed analysis.