You said it yourself. The architect screwed up, and that is why he has errors insurance... to pay for mistakes if and when he makes them. I would get an attorney involved, and have him send the architect a registered letter advising him that he made a mistake, and he is responsible for the cost of fixing the mistake. His insurance carrier will be paying for the PE to assess the damage, and how to correct the mistake. Like everyone has said, this isn't going to be cheap, but if it isn't corrected now, you will never be happy with the results later on.
When I was building my home, I found a mistake, and the framing crew foreman, said it is a small mistake, and that I should live with it. I said no way, and he replied that he wasn't going to fix it. I told him that he had a choice to either stop work immediately and leave the job, or to tear it out and fix it to conform with the plan. He and the rest of the crew walked off the job. I called my attorney, who called his boss, and explained what had happened. They were back there the next morning, and fixed the mistake. All too often, the people that are doing the work, have the attitude that they won't have to live with the mistakes, so there is no reason to fix them to satisfy the customer. The only thing that changes that is when you, the customer insist that it be done.
