Duncan - I agree that your idea of a collaboration was kind, thoughtful and clever. It does seem like a good arrangement all round.
I think Mr. Stewart is not seeing the opportunity here. If the Tobermory Cat becomes even more famous and popular through Ms. Gliori's book, it can only increase traffic to his Facebook page, and sales of his postcards etc.
If Mr. Stewart does want to make money based around the idea of the TC, I believe he might need to rethink his plan. If I understand correctly, he intends to make the cat famous through Facebook and then somehow transform that into licensed marketing.
I can't see how that would work. Just as he can't prevent anybody from writing about the cat, he can't prevent anybody from taking pictures of it either. So unless he's willing to sell his images very cheaply, what's to stop a company sending someone out to take their own pics of the cat with their sign in the background? Mr. Stewart might take a dim view of it, but I'm pretty sure anybody can also draw their own version of the cat, write "approved by the Tobermory Cat" on their website, and so on.
My point is that Mr. Stewart has no legal entitlement to the name "Tobermory Cat". I understand that Mr. Stewart is a talented artist and a film maker. It's a shame to see somebody so creative wasting his time and energy trying to hang on to something that doesn't belong to him. If he's still inspired by the cat's spirit and boldness, it seems that he has many ways he could express that that could be more profitable.