"Important factors like availability of guns... are forgotten"
Ah? Well, let's address that immediately:
Given that the stated intent is to keep mass-murderers from getting guns, how would you change the availability of guns to have the desired effect?
This isn't even about mental health, which the author puts forth as a separate issue. It's about preventing a future mass-murder by preventing access to guns for someone who may not yet have committed any crime. Absent the psychic psychobabble of Minority Report, we're left with reducing the number of available guns.
How?
Guns don't evaporate. Hundreds of millions are already legally owned in the United States. If you want to reduce that number, you have to confiscate them, something gun-control proponents vehemently deny any intention of. In fact, they laugh off the very idea.
So if you're not going to confiscate guns, what good would it do to ban their sale? How well has that worked for drugs? Aren't we routinely informed that banning drugs merely pushes the existing demand into black-market routes? If someone can cross the US/Mexico border --- one of the most porous in the world --- carrying 50 pounds of marijuana, what's to keep them from carrying ten handguns? "Dime bag and a Magnum, please."
I find that the "availability of guns" mentality has a distinctly European flavor. Great Britain, for example, is often held up as a model of gun control Americans should look to.
An island nation with one of the most powerful naval forces in the world, and a total land area which could fit nicely inside the US state of Texas. It's rather a bit easier to patrol the Channel than the Mexican border... in fact, it would be easier even if the Channel were completely filled in to form a land bridge. And even if that were the case, none of Britain's neighbors are particularly known for their violent gun-toting drug cartels.
In practical terms, the only way the US could reduce firearm availability to any significant extent would be to ban most if not all of them, confiscate anything not approved by federal authority, and put a giant wall on the Mexican border, treating anyone approaching the same way the British Navy treats unannounced visitors sailing for its beaches.
Welcome to the Police State everyone says they don't want.