I'm not sure what "fake news" or FBI leaks you are referring to. It is true that polls show a narrowing of Clinton's lead over Trump after Comey's announcement to Congress about re-opening the email investigation. Obversely, because of Russian interference with the release of DNC emails (with which the Trump campaign may have colluded), Clinton didn't get much of a bump after the release of the Hollywood Access video of Trump bragging about sexually assaulting women. Again, this wouldn't have happened if Sanders had been the candidate.
But certainly Sanders would have suffered other slings and arrows. Every instance of Sanders saying or hinting of "socialism" or its cognates would have been in every ad for Trump. It's just not a popular word among Americans. One of Sanders' major themes during the primaries was the "disaster" caused by the Citizens United decision, though he did welcome the nurse's union "sponsoring" his campaign, made possible by that decision. He probably would have welcomed the advertizing of other super PACs if he had been the candiate--but if he didn't, or if other super PACs didn't support him, his numbers probably would never have been as high as Clinton's. Certainly the Trump campaign or super PACs would have pointed to his hypocrisy about the nurses' union, in addition to the fact that the decision is law and there is no realistic way to change it. The issue of Sanders' wife (can't remember her name offhand) and the handling of the college (can't remember the name offhand) would certainly have gotten tremendous press if Sanders had been the candidate. She is under federal investigation, and it doesn't look good for her.