The blog of the Austin bomber is a good example of all the "red flags".
As for the argument that the ban is a bad idea because it is ineffective. I don't know if the ban will help. I'm not an expert, though it seems pretty obvious that our current situation isn't working so well. Do you think a ban will increase gun violence? How sure are you?
The suggestion that Youtube should wait until there's proof that videos they host cause harm is nonsense. A private corporation needs to be cautious otherwise they're opening themselves up to unnecessary risk from being sued by a plaintiff who manages to demonstrate harm (within the limits of a single specific case). YouTube doesn't have a moral duty to uphold freedom. They have a duty to protect their shareholders value.
The government shouldn't ban things until there's proof that the thing is harmful to society (and even then there's a good argument that they shouldn't), but that's got nothing to do with what's happening here.
You're presenting a false dichotomy. I have seen no proven link between YouTube videos showing proper gun handling and gun violence. I say this as someone who hates guns, but most of the points you raise are not logically sound.