It's bizarre that you are claiming that the fact he attempted to do the impossible somehow absolves him of some responsibility, or that the responsibility is shared.
It's common sense. You can't fault a party for failing to do something impossible.
No, but you can fault a party for making an impossible claim. If I lie to you, are you equally at fault if you believe it? Maybe you do believe that, but I don't. I think the liar bears the moral responsibility for lying. Maybe Harnett didn't deliberately set out to deceive anyone, but he made various untrue statements. He made the statements. He bears the responsibility for making them.
In this case, there was a shared mistaken belief on which the agreement was based, without which neither party would have entered into the agreement.
You speak as if this shared mistaken belief came out of nowhere. It came out of Harnett's mouth. Yes, it was a shared belief, but if Harnett didn't make the proposal then the deal wouldn't have been made. Harnett made various assurances. He assured the impossible. We can't blame him for not making good on his impossible claim, but we can fault him for making it in the first place surely?