How mainstream was the belief that Genesis is not literal?

Well, the three authors that I listed are among the most influential early doctors of the church, so their views are definitely mainstream (albeit not universally held).

You can't use that as your criteria for "ancient tradition" without making the whole concept of "ancient tradition" meaningless because now everything is one.

I don't know about that. I listed three very influential early Christian theologians who took much of Genesis to be non literal.

how exactly do you figure out which tradition is the "real" one

Your point that there are divergent views on the matter of how literally to take Genesis is certainly true and not in dispute. I alluded to that fact in my post when I said:

it is certainly the case that some believers traditionally interpreted Genesis literally (and some still do)

However, I don't see how that conflicts with my point that one can interpret the serpent story metaphorically without breaking with early mainstream Christian traditions. Moreover, you wrote:

For that matter, since religion is supposed to provide eternal truth, the idea of having a minority tradition in sometinng seems problematic--how exactly do you figure out which tradition is the "real" one...?

I don't see how the fact that there are divergent interpretations of some scriptural stories is particularly surprising or problematic, unless you are trying judge ancient religious texts against the stylistic standards of modern historical or scientific writing (which presumably most people would not recommend doing).