I agree. First, get a second opinion in writing. Second, work with the two agents because they both had an important part in this transaction. They may also be able to help come up with a win/win solution. If you can work something out directly with the seller, that would be the best route. No need to necessarily mitigate unless the sellers feel they aren't at fault or aren't willing to help.
At this point, we don't know what the sellers did and didn't know. It's possible that they hired someone to fix the issue and that person did a sloppy job by just covering up the problem. Perhaps that's just wishful thinking, but it seems as if you have a good argument for them helping with at least part of the repair cost.