Actually, your post 17 refers to the inability to share intel on any grounds at that time due to the Gorelick wall, not any kind of diplomatic protection.
Similarly, if NSA snoops something illegal on you, they can’t give it to the cops to go nab you.
But it doesn’t stop the cops from finding out on their own that you’re up to no good and going after you.
So lack of intel sharing doesn’t stop INS from pursuing expired visas and forwarding their own case to the FBI if they think there’s more going on there than an inattentive student.
It referes to independantly developed intel that could not be used against the protected person of Mohammad Atta. Gorelick’s Wall was not the issue at hand and played no role here.