1. 'agrumentum ad ignorantum' is used everyday as a part of scientific discovery.
So you think science is using a fallacy to derive theories? To make the ramification clear: (I am using exaggerated examples that are more likely to be in science and to make my point clear)
"There is no evidence for aliens, therefore aliens don't exist. Therefore we will not look for alien life because we know it does not exist."
"There is no evidence for ghosts, therefore ghosts don't exist. Therefore we will not investigate the claim that a house is haunted because we know that it cant be."
Or a few hundred years ago:
"There is no evidence for (the idea of bacteria), therefore (the idea of bacteria) don't exist. Therefore we will not investigate (the idea of bacteria)"
2. One does not consider a possible cause for which there is no evidence as a valid contributing factor.
I agree!
3. One always remains open to new evidence.
I agree! But how can you say that I you are sure that fairies don't exist? Or are you not really sure?
How can the label of atheism possibly be unscientific when it is applying part of everyday scientific method?
It is? What you are describing is Gnosticism.
Also Atheism is not unscientific nor is it scientific. It is lack of belief.