I do not quite agree with the contract being risk neutral. The contract implicitly allocates the risk on Patrick's side by promising only a fixed and capped rate. In the opposite event of this business going better than expected there'd be no doubt the excess goes to Patrick for the risk taken.
If you believe the contract implicitly allocates that risk to Patrick, then equitable mistake wouldn't apply. I don't think it allocates the risk that a large number of loans will fail at about the same time due to a hidden interdependency. I believe the contract was premised on a shared understanding that the loans were fundamentally sound.