Sure thing, that sounds like a completely foolproof plan no doubt.
You kinda remind me of the folks who said after 9/11 that we should put anti-aircraft guns atop NYC skyscrapers to prevent the next attack. You both seem to approach problem solving the same way.
Who said it's fool proof? That's the typical gun nutter response: jump to extremes. We shouldn't have seat belts at all because they're not foolproof and you can still die in a car wreck. You shouldn't exercise because you can still have a heart attack or get diabetes despite being thin and in good shape.
Nothing is fool proof. I'm sure 3 nutbags can get together and write each others letters, or a nutbag can threaten a family member to write a letter, or a nutbag may be able to get through a 20 min interview with a police officer.
The point is not that its perfect but rather that
1) its an impediment that'll make them really think about what they are doing
2) it'll catch some if not most of these guys
3) its a start. Its a change; a step something that you can say "hey we did something and it worked a bit or didn't work but we learned something"
Everyone of these mass shooters: loners, white, weird personalities, no friends, family had serious concerns, high chance of being already known by the police, crazy social media profiles, etc etc The basic point is people who know these individuals are in the best position to keep guns out of their hands, as opposed to systematic screening systems like background checks (which for many of these individuals is negative).
B-b-b-but the burden!
And what next? Psychiatric exams for voters! I mean where does it stop?!
Oh? It stops with guns? Oh.
Try that with voting and 99% of the US will revolt.
Try it with guns and likely only the 3% that own 50% of all the guns will revolt. I could care less what those guys think; they're the ones doing all the shooting.