What happens if we put the question of whether your model is itself valid, through your little model?
Knowledge isn't a house of cards with an identifiable foundation. It's a geodesic sphere: every piece of validated objective empirical evidence depends on another piece of validated objective empirical evidence. The overall system is only "valid" because it's internally self-consistent. It doesn't rest on anything external.
I had thought that part of the definition of "skeptic" is the idea that the skeptic is looking for evidence and will rationally weigh and consider evidence offered.
To True Believers - e.g. the folks over at Evolution Fairy Tale - "skeptic" apparently refers to people who accept evidence over the Bible.