A WWP encoder (CCed) was kind enough to read through section 22.5 "Documenting a Processing Model". Here are her comments. Note that she was reading version 2.8.1a, last updated on 2015-12-17, revision 3df969c (which I generated from an updated pm branch on 2016-02-22).
1) My initial question in looking at this was, “what exactly is a processing model?” - this sentence in section 22.5, "In this case a processing model is a template description for how the element should be transformed in one or more particular outputs” confused me slightly because it implies that the definition of a processing model changes depending on its “case” or use, which you explain further here "This is a method for documenting one or more possible processing models in abstract terms and it should be clear that processing of such documents is not limited to only these possibilities." Nevertheless, even though you want to leave the possibility open for multiple processing scenarios, it might be helpful to have a more outright definition of processing models from the beginning. If your target audience is any TEI-interested user who is exploring the guidelines, not just those familiar with processing models, it could be worth opening with a more standard definition.
2) Since this larger section (22.5 - 22.5.5) is about the output for TEI texts, I'm wondering if the section should appear towards the end of the chapter - after the sections on schema development? Basically, I wasn't sure why this section appeared in middle of the chapter after the "Common Elements" section - though that could be because I have only a rough understanding of processing models. ... Hopefully these notes / comments are helpful and are what you were looking for. Overall I found this section very approachable in terms of pacing, coverage, organization. Let me know if there is any other way I can help.