“After all, we spend billions to combat Extremist Islamic Terrorism in the US, famously banning people from 7 Islamic countries, when the number of people who have been killed by citizens of those countries in the US in the past decade is, exactly, zero.”
3000 Americans were killed in the twin towers by Saudis, citizens of a supposed Ally: why isn’t Saudi Arabia on that list? But people from those 7 countries have committed terrorist killings in any countries they have access to: France, Germany and the UK for example. Why would you let them into the USA? The President is sworn to protect the American people from enemies: Donald Trump has done what the Constitution requires him to do.
What should we do about mass shootings in Schools? if I understand you correctly,“ It is a relatively small risk. By arming 10–20% of public school teachers to combat it, you are adding 700,000 guns to classrooms around the country,” your answer is “Nothing.” Those 700,000 guns in classrooms might cause half a dozen accidental shootings per annum, but could save 25 children who die each year through mass shootings.