Is federal coverage for abortions negotiable? I'm sure the Democrats would love it if it was. Are higher taxes on the wealthy negotiable? Again the Democrats would love it if it were. Is ending Obamacare negotiable? From the Democrat point of view I'd say no it isn't. Same with raising taxes on the poor and middle class. How negotiable is getting Mexico to pay for the wall? From Mexico's standpoint it's a non-starter; how do you negotiate with someone who sees no reason to come to the table.
There are certain conservative principles that should not be negotiable. Smaller and less intrusive government. Respect for the Constitution. A judiciary that holds to the Founder's view of what the Constitution intended. But Trump is a man without principles, at least none he would go to the mat for, so he wouldn't know that.
Everything under the sun is negotiable, that is the way of the politician. Now, negotiable does not mean you will cave to the negotiation. Let us take your best example: Ending Obamacare.
Trump can say, "That is negotiable."
What he can MEAN is, Obamacare can stay, if "Obamacare" comes to mean a return to the old, pre-Obama system and a destruction of state-lines that caused the insurance problem to begin with.
When you are a politician, EVERYTHING is negotiable. It merely depends what your goals are, and how good a negotiator you are.
I actually believe that this 'leak' from the NYT was coordinated, by Trump, to signal the Elistist/Globalist party not to fear him. However, when he is in office, he might not be as much a pushover as they think.
We cannot know for sure what is truly happening in politics these days, except to know two thing: There is an Elitist/Globalist cabal that has seized control of both parties in America, and there is a probability that Trump is challenging that.