What would have happened differently if Nancy wasn't so protective?

I think if she kept him in school he would have gotten okay social skills (for an aspie), and maybe would be okay.
I remember in the summer of 2015, my parents weren't home alot, so i spent hours in the basement, not getting any human contact. I pulled out of reality, and started hating people.
Luckily I'm fine but I think AL took the same path.

I'm not convinced school would have been the best environment for him, but I agree with the basic premise of a positive social life. Even in a homeschooling environment, there are ways to make sure that social needs are met, such as team sports, churches, homeschooling co-ops,spending time with kids of similar ages in the neighborhood, spending time with friends of the parent's children, etc. Had she taken his socialization more seriously, things may have been very different, but then again, maybe not.

Unpopular opinion: I do not think there was anything realistic that could have prevented the shooting. (When I say realistic, I mean situations that would have been obvious)

Should Nancy have paid closer attention to Adam and his problems?
Yes, but she was not that type of woman. She thought she was doing enough.
Would staying in school have helped Adam?
Possibly, but he would have likely talked Nancy into letting him leave sooner or later. I believe he had it in his mind that he was destined for isolation.

The shooting in my opinion was entirely preventable, there were countless opportunities for Adam Lanza to get medical help but his parents, more specifically his mom, time and time again kept coddling Adam and not listening to the psychiatrists at Yale and instead insisting on only sending him to that one psychiatrist who pretty much did what Nancy wanted, which was to pull him out of school completely. Had Adam spent the whole time in school and not kept being shuffled around and sent in and out he would have likely overcome his anxiety along with being prescribed suitable anti anxiety medications. The problem was Adam was in general against medications and so was his mom, especially after the incident when Adam "could not raise his arm" after taking a low dose of a prescription medicine. The last major point at which things went down badly was when he was pulled from his highschool. Had Adam Lanza met someone who was close by and could have been a good friend I suspect the time line of Adam's life would have shifted. (I have a sneaking suspicion Adam Lanza faked that adverse "symptom" so he could get off the medication and not have to take it) I myself feel bad about his situation and life, had I lived around that area, I likely would have been able to help him come out of his shell.