I don't think that motivation on the negatives helps as much as it probably should. Trump and Hillary both have astoundingly high negative numbers and I think that generally will turn voters off and favor the Republicans slightly, IF (big if) the RNC decides to get behind Trump.
Negativity will drive people pretty hard. "Anyone but Trump" actually made people willing to vote for Cruz.
Let's face it, negativity was what started the antiestablishment movement.
You think the ghost of Bush didn't help propel Obama to victory? Don't forget the 2006 midterms Bush's general ineptitude helped Dems prevail, without any charismatic figure from the left to help. The Republicans manage to poison their own brand, and the democrats were there to clean up the senate seats.
I have zero doubts that this general will not favor the GOP, because Trump has alienated everyone except his angry white man base.
Now 60 million Hispanics have a pretty great reason to flock to the polls, and if anything a moderate like Hillary would probably be more appealing than a dedicated leftist like Bernie.
I think what you underestimate is just how out of touch Trump is with the mainstream. That's why this race has taken everyone by surprise, the GOP primary has failed to function as a proper vetting process. Thanks to radicalism, they've put forth their least electable candidate whose own rhetoric will have poisoned his campaign.