* The ban is a temporary measure and has nothing to do with religion. President Obama signed a list of countries into law for enhanced visa scrutiny and President Trump used that exact list without modification, directly referencing the statute in which it was codified. They'd already been identified as terrorism hotspots that warranted additional scrutiny from immigration officials by the Obama administration. Trump just took it a step further and temporarily halted immigration from these nations while his administration reviews the policies to ensure they're sufficient.
* Alien permanent residents from the affected countries are not currently banned. They never were. Neither are dual citizens.
* It is true that non-diplomatic visas have been temporary suspended (not canceled), but this applies at border crossings only; persons currently in the country on those visas do not have to leave.
* There has been 18 months of warning in the form of the Presidential campaign. Trump promised something like this up and down. He was taken seriously, not literally, and he seriously implemented a 90-day stay on the travel of foreign nationals from areas with high terrorist activity, using the exact list that President Obama signed into law. He did not literally implement a ban on Muslims, or indeed, any other religious group, despite the literal meaning of his prior comments that did explicitly mention Muslims. Likewise, his statements about "the wall" seem to be flexible to the point of something more like multiple layers of well-patrolled fencing.
To me, "seriously but not literally" appears to be holding up really well. Trump is not betraying the intent of the voters who elected him, but he is also consulting with his advisors and legal experts and implementing things that accomplish the intention within the constraints of practicality and legality.