The whole movie is one big PLOT HOLE
Ok try to keep up now.
First off, Doc has already been to the FUTURE. He knows what is going to happen to Marty and his family on October 21, 2015. He knows that Marty's family will be destroyed due to Griff forcing Marty Jr. into the robbery. So Doc goes back to 1985 and warns Marty about the future.
Ok at this point this is all Doc had to do, warn Marty. There was no reason to take Marty (and Jennifer) back to 2015 and change anything.....because it's the future, NONE OF IT HAS HAPPENED YET. Doc could have told Marty, " look I've been to the future, one day Biff's grandson is going to force your son into helping his gang in a robbery and your son and daughter will end up in prison and who knows what else for your familly."
So now with this information about an event 30 years into the future all Marty has to do is make sure his son doesn't go around Griff that day and go along with the robbery as well as warn the police that day that a robbery is going to happen and tip off the cops about who is involved.
Second, if Doc is so worried about Marty's family being ruined by Griff and the robbery why not go to the cops himself while he's there in the future and warn them about the impending robbery?
Third, by the time Marty actually becomes old Marty and that day in 2015 arrives....would there be two Marty's and two Docs since Doc already arrived there the first time?
Fourth, the whole point of going into the future becomes pointless anyway by the end of Part III. Since Marty doesn't crash into the Rolls Royce and doesn't injure himself his entire future will change so most likely Marty Jr. will never be there for Griff to threaten him into the robbery thus Doc need not persuade Marty to help him make sure it doesn't happen because it wouldn't anyway.