You seem fixated on the definitions within the Decentralized Virtual Currency section. What about the definitions that are present elsewhere, like this one:
"An administrator or exchanger that (1) accepts and transmits a convertible virtual currency or (2) buys or sells convertible virtual currency for any reason is a money transmitter under FinCEN's regulations, unless a limitation to or exemption from the definition applies to the person."
Well that is the entire point. This "guidance" on virtual currencies is incompatible with the specific guidance given for decentralized virtual currencies (which of the three subclasses is the only one even remotely applicable to Bitcoin).
It is possible since this is just a guidance letter than the phrasing used in the "section c" is an error however it warrants clarification from FinCEN rather than an assumption. However the general definition is overly broad. It would by the letter of the guidance make anyone an "exchanger" even if they use a licensed "exchanger" maybe that is FinCEN intent but once again it seems excessive and vague.
If FinCEN wants to define regulation then they are obligated to provide clear and exact lines. Regulation which is vague, arbitrary, capricious, and overbroad is generally overtuned. Now FinCEN may lack sufficient understanding of the mechanics of Bitcoin to properly define those lines and hence provide conflicting and confusing guidance but if they intend to regulate it they damn better understand it in order to answer very exact questions on which activities fall inside or outside of their scope.
I disagree that the different sections of guidance are incompatible. If ANY of the definitions apply to you and there is no exception defined, then you are covered. You can't cherry pick parts of the guidance.
The guidance doesn't make "anyone" an exchanger because there is an explicit exemption for "users" purchasing goods and services who are not in the business of buying and selling currency, so I disagree on that point as well.
Also I disagree with your last point. The guidance is not that bad from my point of view. They are capturing the activity they want to regulate with language that is about as precise as I would expect from a regulation of this sort.
The only vague definiton in my view is whether a miner or mining pool who confirms transactions is a money transmitter. The question is whether a miner "accepts such de-centralized convertible virtual currency from one person and transmits it to another person as part of the acceptance and transfer of currency, funds, or other value that substitutes for currency." It is hard to see how miners who confirm transactions avoid this definition. And they can't use the purchasing exemption that regular "Users" could to avoid regulation. You can ask for further clarification on this but you may not like the answer.