I’m not sure how that related to your original question. If your boss is instructing you to do something that you deem to be unacceptably risky, make sure they are aware of and take ownership of the risks. Large and especially regulated organisations (like banks) will have processes to track things like risk ownership. If you communicate risks like this, then not only have you covered your own ass, you’ve also done the responsible thing of notifying the correct stake holders. If they choose to act irresponsibly in light of that, then that’s on them.

I’ve done a lot of work in banks and this story sounds very, very familiar to me. Change the details of the system and it could be any number of projects that I’ve personally worked on. I’ve been brought into a number of projects like this at the ‘near completion’ stage, and each time I’ve reported on what I thought the risks were, suggested how they should address them, and advised them to delay delivery until they do. Some of those projects worked out well, some of them completely bombed, but I’ve never been put in a situation where I was made even partially accountable for somebody else’s poor decision making. So if your question is “how do I protect my own interests when my boss wants to be an idiot?”, then that’s the serious answer for how you do it.