Negatives on Trump are likely to stay high until he formally wraps up the nomination. Once he becomes the clear alternative to the coughing and hacking Hildebeast, we will see those favorability numbers start to move more in Trump's favor.
Still, it's way too soon to get a good grasp of the general election. First things first, let's get Trump nominated and put this nasty business of Cruz/Kasich in the rear view mirror.
I think tomorrow night's results in RI, CT, PA, DE and MD will put an exclamation point on Trump's inevitability. If Trump then wins IN a week later, he is on a glide path to the nomination and I believe we will see Cruz drop out as his funding will dry up quickly.
I believe that the sequence is as you describe. Once freed from having to endure the idiotic attacks from other Republicans in the Primaries; Trump can show a more positive image. I continue to urge this approach, as a major tactic, believing he has the aptitude and personality to be extremely effective in the type of speech proposed:
http://www.freerepublic.com/focus/f-chat/3422096/posts
If Trump gets the Republican nomination and is in a debate with HillaryI hope that he has a handkerchief. Imagine the optics if Trump to offer her a handkerchief while she hacks her brains out. It would be good optics
The trouble with C/K right now is it’s only real purpose is to trash Trump and, in the process, create a Three Stooges spectacle that will devastate whatever hope the Reoublicans have of winning the 2016 election, a catastrophic loss that will enable the Witch to gut the Constitution by appointing up to four radical SCOTUS justices.