Due Diligence; what was done exactly?

I wrote before; "I can't wrap my head around the DD myself. I mean S&G had 70 in house lawyers reviewing PSD case files and S&G worked with EY and other independent advisers who gave the green light. Chairman Skippen said it was the most thorough DD he had ever seen. SO WTF HAPPENED HERE?"

Mighty replied besides S&G the following were involved in DD:
* Citi
* Greenhill
* E&Y
* PwC

I would like to know exactly what each of the above did to recommend S&G purchase PSD. It seems to me there are many questions to be answered about the DD.