Again, this brings up the question of why this wasn't pursued by her parents at the time of her injury. Then, after being taken to the emergency ward, her case was mishandled, leading to the lawsuit which is where the money came from.
I would venture that there was too much going on when Terri collapsed(daughter possibly never to recover, hospital screws up, lawsuit to enter into, etc.) for them to spare time to pursue a possible case against Michael. They may not have have any of the information from the EMT's, etc. that you are talking about. So they may have been suspicious, but no clues were available.
What I have wondered is why her brother, who stated that Terri told him she was thinking of divorce, wasn't suspicious at the time he says Michael called him over.