I think rational proof is there, but it will be subjective, not objective. As many have noted (such as Dostoevsky and Douglas Adams), objective proof precludes faith, which is an important way of approaching God. However if the Christians are right, rational proof in the form of God's firsthand word is written on the human heart, and accessible to everyone.

It's a matter of politics. People long for simple answers to hard questions, and pseudoscience is the modern equivalent of alchemy and magic. Most Americans are not good at science, but would never admit it, because they need to believe they're "smart." Whether someone chooses to believe in creationism or Whole Foods silliness is largely a matter of political preferences. It's frustrating because the concept of the scientific method is easy to understand, and should be learned in school.

It doesn't make sense to choose an arbitrary number, the central question is: Is it ruining my life? If some is on the Internet for every minute of their spare time, and the overall quality of their life is greater, or about the same as before the Internet, then no medical illness is present. However, if another person is failing to go to school or work, losing important relationships, and suffering other serious negative consequences, AND they cannot stop in spite of all the loss, that is probably an illness. Illness doesn't mean excessive use, it means very serious consequences, up to and including death. This is an uncommon illness, but probably not rare.