I am not arguing that the terms of the contract specified a share of the losses. I'm arguing that the losses occurred because of a mistake and the harm from that mistake should be born by those who made the mistake and caused the harm.
How did the people who deposited with him cause harm? Yes mistakes were made all around, but Patrick's mistake was the only one that resulted in a loss of other people's money, which he promised to repay on demand.
If we assume that Patrick is obligated to repay them from his own funds, then Patrick was harmed -- with that assumption, the investment resulted in a loss of Patrick's money. Patrick is a person other than those who loaned him money. You can't use an argument like a bus and get off at your stop -- you have to see if it ends with consistent results and, if not, keep going.
If you start out trying to enforce the contract as agreed and think that means that Patrick is 100% responsible, then Patrick next has a claim against his investors for the harm their mistake caused him. This claim would offset their claim against him for making the same mistake. You end with them needing to split the losses.
However, I think seeing it as common mistake is more sensible because I don't think it's possible to enforce the contract as agreed at all since the agreement was about a loan portfolio that didn't exist in the form the agreement presumed. But you get the same result either way. It depends on how you try to construe the terms of a vague agreement.