IMO, the strongest reason why I believe Doc knew/ed everything is because he was not a creep... An old man becoming really close friends with a teen is creepy unless the scenario that Marty found him in the 50s happened all along. I would love to see/read how Doc might have first approached Marty and how their friendship in the 80s first evolved (I recall Doc sayin I. One of the films that he would have to wait 30 years to tell him what happened in the next few days, so it's always about Doc restraining himself from over sharing so that the time travelling events would happen). Also, Doc explains the bare minimum to Marty as for him to be able to convince 50s Doc that it's all real. And it also explains why Doc lost all his family fortune from the beginning... He knew it would work because of Marty so he went all in trying to succeed.