Actually, what would have happened- what should have happened, is that the minute she moved to have her son committed to a mental institution, the Sheriff should have come around to collect all her guns. Period.
To me, it is inconceivable that when you get someone who was so deranged his own mother wanted to lock him up, but they let guns remain in the household.
The only list that has a snowballs chance in hell of working at all is a publicly available "Registered Disarm List" just like the "Registered Sex Offenders List"? The list would contain domestic violence aggressors, violent criminals, anyone prescribed psychotropic drugs, unstable persons reported by doctors, nurses, & psychologist, etc.
Friends, Family, co-workers, employers, law enforcement & business could have checked this list on their smart phone & reported when they saw Adam Lanza with a gun or practicing shooting at the gun range. An employer would know who might go postal & shoot up employees at the workplace when fired. Citizens could check the list if they sell, lend or trade a gun to someone else.
The only thing that stops a bad guy with a gun is a good guy with a gun. The only way to find out if a bad guy has a gun is publicize a list of potential bad guys so good guys can keep an eye on them.
Gun free zones are a magnet for crazy people with a gun. Limiting good people from conceal carry in any way puts everyone in danger.