Can Someone be Simply "An Agnostic?"

[Rhizomorph13]

I'd like to know: if agnosticism isn't a tenable position, why does ignosticism exist as a concept?

Ignosticism is simply a re-branding of theological noncognitivism which says that the concept of God is so poorly defined as to be a meaningless concept and until someone presents a rational definition of God there is no need to even begin the discussion.

Is it possible to be opposed to irrational beliefs, and be actively critical in order to avoid them?

Yes.

Is a belief for or against a proposition for which tenable evidence for or against is impossible, strictly rational?

Yes, quite easily like the laws of logic. They are properly basic and you can't prove them without resorting to them as the structure for the proof.

If we can agree knowledge is a justified true belief, and beliefs are independent of knowledge; no matter how strongly a belief is held it only qualifies as knowledge if it is justified, is it strictly rational to hold beliefs for which justification is impossible?

Depends on what you mean by justification. According to what I think justifies a belief... no a belief without justification is not rational.