Can changes my Dad made to his estate plan be invalidated if it's proven he was mentally ill?

My question really is: If an elderly person's psychiatric records prove (and document) that the person was mentally ill and/or delusional at the time that they changed their estate plan, can the changes to the estate plan be invalidated upon proof, even after the elderly person has passed away? What is the process to get the proof and then have the changes invalidated? My severely mentally ill and delusional dad changed his part of my parents' long-standing estate plan. Because the standards in his state for proving mental competence are very low (does the person know their name, address, phone number, have a sense of time and place--- basically a mini-mental test), his estate atty did no more than this cursory check, which he passed. His atty was unaware that my dad was under psychiatric care at the VA in his city. Had she legally been required to also have 2 drs certify his mental competence, in addition to the mini mental test, and/or to have him seen by a Psychiatrist to assess his mental functioning or checked with his Psychiatrist, I believe that my dad's atty would never have allowed him to change his part of my parents' estate plan. He unsuccessfully tried to change my mom's part of the trust, but legally couldn't because she had already passed away (and even if she had been alive, she had dementia and wouldn't have been competent to agree to the change). My dad has been very successfully able to hide his long-standing mental illness and fairly recent delusional behavior from non-family members. His atty remembered him from 8 years previous when my parents had changed their estate plan and when my dad wasn't delusional. Even at that time, he had hidden his severe mental illness from her. So, she appears to have just gone on what she knew about him before. Given his pathological lying and his ability to manipulate and blindside people, it appears that he did this with his atty and she allowed him to change his estate plan. In my opinion, when someone in their 90s suddenly decides to change their estate plan, this should raise all sorts of red flags in an estate atty's mind. And, whether or not it's required by law to do so, the estate atty should do more than just a cursory mini-mental test and should be go the extra mile to contact the elderly person's drs to certify the person's competence and should also seek to find out if the person is currently or has been under psychiatric care. If not, the atty should request that a psychiatric evaluation to determine mental competence be done. Until drs and a psychiatric eval have certified an elderly person's mental competence, the estate atty should allow no changes to the estate plan to be made.