You are technically correct that most people that call themselves atheists are agnostics. But that's a little like saying that .999999999999999999 is 1; it's close enough that it's not going to make a difference in almost all cases.

If you have empirical, verifiable, falsifiable evidence that a god exists, then most atheists will change their opinion. A person that believes in a god believes despite a complete lack of evidence.

It's not that an atheist is saying, "I know your religion is wrong"; it's much closer to, "you have not presented any falsifiable evidence that your religion is correct.

There is currently no way to know.

It's true that you can't prove a negative. On the other hand, no evidence exists that would tend to prove that a god exists. The lack of evidence is quite damning, particularly since people have been trying to demonstrate the existence of a god for well over 8000 years. Miracles have almost entirely ceased in the age of forensics, modern medicine, and photography; it's almost like they only exist when they can't be documented.