Well, I am surprised this got resurrected.
As for errata vs. FAQ - I'm afraid that the FAQ acts as a virtual errata for WotC, not just interpretations. This is not according the their own original rules, but it's what has happened over the years. Some items from the FAQ (never published in errata) made it into the latest printing of the DMG/PHB. Sorry, but I cannot think of an example off-hand but I have actually seen at least one example of this.
Therefore, if errata can be used to support RAW arguments I suppose the FAQ can as well.
In this case, I (and others) have shown (repeatedly) how the rules as written (without errata or FAQ) support the position that monks can take INA - and how that position is closer to the rules as written than not allowing it by two completely independent streams of logic. I personally have also admitted that the minority view could also be valid, even if it really is not the correct position

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I have nothing new to add, really, except the comment about the FAQ being treated as a virtual errata list, which is unfortunate - it never should have evolved that way. The result is that it's a little hard, sometimes, to figure out if a particular FAQ item is:
1. An official clarification/interpretation.
2. Merely intended as good Advice.
3. An offical, actual rules change.
In this case, I think we are talking about an official clarification/interpretation because there are indeed two legitimate ways to view the rules so a judgement from the authoritative source was required.