The key issue is whether they did, or reasonably must have, known. If they did know and failed to disclose, you may be able to hold them liable. If they did not know, then they would not be responsible. The "supportive 2x4s" provides at least some evidence that they did know, but it's not dispositive: maybe a prior owner put them in; maybe they were there to remediate some other issue or provide general support (i.e. no connection to mold). For the amount  of damages (money) you describe being at stake, it is worth your time to consult in detail with an attorney to discuss the situation and evidence and explore your options.