And what about the inverse? If he is Satoshi, why didn't he just put out a signed statement saying he is Craig? Satoshi seemed perfectly capable of producing valid signatures (and he could do it without a 25-page blog post).

Stop giving him the benefit of the doubt. He couldn't produce a valid signature. He couldn't move old coins. He couldn't explain the blockchain with anything more than an amateur understanding of it (his article describing mining had at least 2 major factual errors I could pick out in the first 2 paragraphs). When he realized he couldn't keep the charade up, he gave up. It's really that simple.