Mike,
I'm not sure that you can build a case that Gen 15:6 is a "conversion"
or "conversion-like" experience for Abraham, which is the interpretation
that you seem to be trying to make. The case cannot be made from the
Hebrew, for one thing. The most apparent interpretation of the verse in
that context is that Abram believed God's promise that a son would be
born to him who would become Abram's heir, and that his descendents
would be as innumerable. This believing was valued by God as
righteousness.
Even Paul and James are not making the case that this situation marked
the inception of Abraham's believing. They are, rather, calling on this
passage as a "proof-text", if you will, that Abraham *did* indeed
believe. This is why the writer of Hebrews (who I feel could well have
been Paul, but I will not get into that argument in this forum, since it
really is not that important to me) could describe an event well before
Gen 15 as an illustration of Abraham's faith.
IOW, Abraham *did* believe when God "made the covenant with him" in Gen
15, but he *did not* necessarily *start* to believe at that time.
Sorry if this doesn't give you a warm fuzzy in your inquiry,
Paul