Why didn't Jeremys highly paid defence team get this information from Sheila's social workers.
That is a very good question. However, it does not have any bearing on the fact that Police were able to obtain such information, presumably when the case was being investigated as four murders and one suicide, then withheld the information.
The defence did consult several psychiatrists. None said Sheila was capable of going on such a murderous rampage. One also said Jeremy was a psychopath. Able to block out his crime from his mind.
This isn't my area but I expect it will be challenged.