What about if I bought the MacGuffin for £500 and you value it at £1000? This makes no difference to the formal structure of the scenario (note that me valuing the MacGuffin at £11 would change the formal structure: then I might do X even if I knew Y was impossible: I would still reject an offer of £10). Then my behaviour feels utterly reasonable, rather than vicious and blackmail-ly.
I suggest moving the parenthetical to the end of the paragraph. I had to re-parse that in programming mode before I interpreted the ambiguity in the last sentence correctly and I wouldn't have noticed my confusion at all were I not familiar with the distinctions you are trying to express.
Thanks for the comment - I simply removed the parenthetical, it's not adding much useful in such a short post.
For a more parable-ic version of this, see here.
Suppose I make a precommitment P to take action X unless you take action Y. Action X is not in my interest: I wouldn't do it if I knew you'd never take action Y. You would want me to not precommit to P.
Is this blackmail? Suppose we've been having a steamy affair together, and I have the letters to prove it. It would be bad for both of these if they were published. Then X={Publish the letters} and Y={You pay me money} is textbook blackmail.
But suppose I own a MacGuffin that you want (I value it at £9). If X={Reject any offer} and Y={You offer more than £10}, is this still blackmail? Formally, it looks the same.
What about if I bought the MacGuffin for £500 and you value it at £1000? This makes no difference to the formal structure of the scenario. Then my behaviour feels utterly reasonable, rather than vicious and blackmail-ly.
What is the meaningful difference between the two scenarios? I can't really formalise it.