My main concern with Trump is the inconsistency. He would go along way in getting the Cruz, Rubio, Paul, etc... supporters by:
(1) Being more specific on policy, and stick to it. Assure the rest of us that he has a real handle on this.
(2) He could point out where he is in agreement with most of the candidates he ran against. On many issues, they were a lot closer than they were apart. Paul and Trump both want to Audit the Fed, for example. Cruz and Trump were both for rebuilding the military, and both tougher on Immigration than most.
(3) Lastly, give it time. This was a tough race, with al lot of bad blood spilled. People will come around. we have six months.
It is tRump's responsibility to be very clear (in writing) on the 'conservative' things he WILL DO, WHO HE'LL PUT ON SCOTUS, HOW HE"LL RELACE NOBAMACARE, ETC. Anything less is just more of the same Snake oil New York Values BS to say one thing and do another.
What has he done since Cruz exited? Still flipping everyone the bird! Not the way to solidify support for a serious campaign against hitlery and her solid MSM PAC.
I will not be surprised if he decides he's showed us all who's the Boss and walks away not accepting the nomination.
Within a sea of childish whining, hyperbole and "take my ball and go home" maturity...your post stands out as being reasonable and well thought out. I agree with you that Trump needs to do the things you mention, and it is incumbent on him to go the extra mile to heal some of the rifts created by his style and...well..crassness (I would call it candor, but I can see why many find it crass). Some of this rift stems from "establishment" resentment that an outsider has taken over leadership of "their" party, but it remains nonetheless Donald's job to bring as many people on board as is possible, including establishment leaders.
For the record, he has been clear as to the kinds of Supreme Court nominees he's looking for...in the Scalia mold. Personally, I'd like see him...as a candidate...put forward that he WILL nominate Ted Cruz to fill the Scalia spot once he takes office. This would go a good ways to healing some of the current rift and hurt feelings AND there is no doubt that Cruz is a brilliant legal mind.
So I just wanted to note that your post stood out as being well considered and well articulated. I think you're overly concerned that Trump will "flake out" on the nomination and that he is on the rude side, but you are right that it is his job to repair that perception as best he can.
Remember, though, that sharp wit and aggressive attack is now shifting towards Hillary...and she has been a given a free ride for a very long time. He is calling her out on her treatment of "Bill's women", Benghazi, playing the "woman" card inordinately, her email felonies, and her horrifically poor foreign policy skills that have put the Middle East into utter chaos (more than the usual madness).
So Trump is still going to be Trump, but I think you'll see him working hard to heal the party's conflicts....in his own admittedly rough way...he knows he won't win over the crazies, like Bigun-liberty-Sink, but he will do everything in his power to hang onto voters like yourself who are at least willing to wait and see how things progress. At least, that is my very sincere belief.