Not sure why you're so one dimensional in your thought process. Hardening of schools can mean lots of things.
Never mind the specific points — hardening schools is a distraction, an attempt to avoid dealing with the real problem. Like Pens pointed out... if tightening security was the overall solution for mass shootings, you'd need extensive security measures in virtually every public space and private business.
It's almost amusing to watch these diversions. 2A absolutists know more gun control is the solution, that all the evidence supports this solution, but they dance around it due to a combination of stubborn pride and reluctance to give up their toys. Like a homeopathy devotee refusing to admit they need a prescription to get better.
Oh, and to touch on your latest statement: no, a "steep slippery slope" is not inevitable. There are countries that allow or guarantee personal firearms that still have more sensible gun control measures. A slippery slope argument is considered a basic logical fallacy in debates; you're operating on fear, not facts.