On the first question, pretty much no. In 2012, we all made fun of Mitt Romney for his many, many flip-flops, but as a Massachusetts Republican, being left of his party was also called "representation," so get over it. Other famous flip-floppers included Ted Kennedy on abortion. Yes, really. Something must have been in the water in Massachusetts. Specifically, dead girls killed by Ted Kennedy in drunk driving incidents that lefties don't like to talk about...
Then, of course, there's this.
Hey look! Massachusetts again! In 2004, John Kerry got himself in trouble by talking about voting for one version of a bill, then against another version. FLIP-FLOPPER! FLIP-FLOPPER! The Republicans started selling John Kerry flip-flops as a fund-raising gimmick. Did it matter? Kerry lost, but let's not fall prey to the post hoc ergo propter hoc fallacy. Yup, that's Latin, kids. It means "after this, therefore because of this." Just because Kerry lost after being accused of flip-flopping doesn't mean he lost because he was accused of it. Incumbent presidents win more often than not. Bush the Younger won. He was the incumbent. The economy was pretty much OK. No big shocker there, from my political science perspective. Did Kerry underperform given the economy? Not really. Do flip-flops hurt? Uh....
Anyway, moving on to the question of whether or not Trump is so closely connected to immigration that his blundering on immigration will hurt him. Great book time. Fred Greenstein's The Hidden Hand Presidency. During Eisenhower's Presidency, a lot of people wrote him off as a little, well, not quite there. If you read the text of what he said, he didn't always sound, well, "smart."
Then again, let's remember who beat Hitler.
One of the great tidbits that Greenstein dug up was that Ike managed to weasel out of trouble by intentionally mangling his language, and counting on the good will he had built up to get himself out of a jam. And, while the text of what he said didn't read well, it sounded fine. And it usually worked.
Is Trump Eisenhower? Um, no. My point is that context determines interpretation. And, Trump blunders so much that his recent immigration stumbles can be written off as just another example of Trump blundering around. He didn't flip-flip. He didn't betray any core principles. He's just being Trump and sticking his foot in his mouth.
Besides which, he's losing anyway, so what does it matter? RealClearPolitics currently has Clinton up by 5 points in their polling average, and PredictWise gives Trump a 21% chance of victory. Notice the lack of movement in response to Trump's immigration blundering. Why? Because none of this matters. I said it back in April, and I stand by it. This is just sound and fury.