My theory on why it has not damaged him is that the crowded field worked to his advantage. When he was exposed in that Fox debate he could always fall back on going after little Marco or lyin' Ted with all the accompanying insults afterwards, and immediately shift the narrative. While she is not a good campaigner (she has stated this on more than one occasion) she is a superb debater and she is going to eat him up and spit him out when debate time rolls around. Will insults work after that, I have my doubts.
Democrats are already setting expectations of Clinton's massacre in the debate so high, she had better be able to live up to them! She won't find this easy though as the rules of engagement are seriously tight in terms of when she's allowed to speak, and the terms she's allowed to intervene on. Don't expect Trump to negotiate an open format
I actually suspect however that we won't see a traditional 'debate' but rather Trump making a sales pitch. He can probably mount a better sales pitch than her. Then we'll see whose ended up making the better impression? He'll go big on sentiment reassuring everyone that he'll deliver them to sunnier upper pastures and that they'll all love it because everyting will be beautiful. He'll also paint a dystpoic picture for those who fail to embrace him. In theory such shallowness shouldn't work, but don't be shocked to find that a helluva lot more people are taken in by it then you'd like admit are capable of being. People will identify with 'Make America Great Again' before they do 'Make America Whole' (I think she's dropped that badly conceived strap line now anyway).
Debating with a populist who is reaching into the camera rather than addressing the podium is never easy. She's got to nail him on substance, he'll avoid that and go big on sentiment. America is the world's archetypal consumer society and this is what Trump will play into. He'll promise to make things better for everyone, she'll end up complaining that he's never said how he's going to do it. Then people decide. You can have Trump's unsubstantiated promises of milk and honey where everything is going to be "beautiful, big and everyone will love it" or you can have more of the same with Hillary Clinton
It'll get nasty of course, she'll accuse him of being a Democrat, and he'll brag about how he's bought her in the past, and comes running to his wedding the moment she thinks she might get some money off him for her senate campaigns and the Clinton Foundation
I think the other thing to remember about his gaffes though is that a lot of them didn't come in the debates, so I'm not sure the name calling as his escape pod really applies to the level you're ascribing to it. Most of them came in interviews, town halls, or on Twitter
The two most crucial ones in the debate cycle were made by Rubio in New Hampshire, which to be honest blew up out of all proportion to what he actually said, and Cruz in Iowa when "New York values" not only came back to bite him in the Big Apple, but also rolled out to the rest of Yankee land
He could always help himself mind you if he actually prepares a bit for a change