I think that any church has concepts that are central or “core,” as well as concepts which are “peripheral,” or ultimately irrelevant to the truthfulness of the religion. If you can prove a core principle of a church to be wrong, then you have in a sense proven the church wrong. However, by their nature, core principles are almost impossible to prove wrong (i.e. it’s impossible to disprove that God exists). That doesn’t mean that you can’t know the truth about religion (I personally believe the best summary of how to learn spiritual truth lies in Alma 32), but it means that you can’t apply usual rules of scientific experiment… which makes sense when you think that a loving God wouldn’t want only his “smartest” children to learn the truth.

Having laid that out, I think that most histories typically fall into that category of “peripheral.” Proving that the church (or any church) has flaws in its history does not prove that the core of the religion is flawed.

So now to answer your questions (my answers in bold)

(1) Am I atypical or abnormal for not basing my testimony, at least in part, on the Church’s historic claims? (e.g., that witnesses actually saw the Gold Plates);
I think that you not in the majority, but not a-typical.

(2) based on your observations, does the Church urge members to base their testimonies, at least in part, on the Church’s historic claims?
I think that the church wants members to have their testimonies rooted in the spirit. Having said that, I think that they do encourage members to believe in certain historic claims, and that certain historical events (i.e. that Joseph Smith saw and communicated with God) actually approach the core of the religion… but the specifics (i.e. which version of the First Vision is the correct one) are peripheral, while the concept (God spoke to a prophet) is the important/core aspect.

(3) when we build our testimonies on our understanding of historic events, are we building our testimonies on a foundation of stone or sand?
Sand.

- Morgan

I find it interesting that you mention your experience at BYU. I was surprised at how often my religion teachers would tackle the difficult topics, such as polygamy, blacks and priesthood, source of P of GP, etc…even if they didn’t have all the answers. there has been sufficient research from both LDS and non-LDS scholars to make an apporpriate judgement on all of the topics you mention.

In the end, it seems like there is more than just a small piece of history or doctrine that drives these people people out of the church.

Has the church “presented them with a sanitized, whitewashed version of its history, and [thus] deceived them by either covering up or failing to acknowledge historic facts”? No, there are public records available to do all the research you want. But what is taught in church is going to be faith-building, not faith-questioning. Why should we spend 40 minutes in sunday school discussing the different accounts of the First Vision? I for one think the time should be spent on the event, its impact, how it affects our own spirituality, and what it means for us (and the world) today. The antagonist will say that that means the church is “whitewashing” the doctrine; the proponent will say “go read about it on your own time, no one is trying to conceal it.”

So, to answer your questions:

1-you are non a-typical. If a testimony is based on historical claims (which are completely dependent on author perspective, since literally no one that is alive today was there at the event), that testimony will surely fail.

2-I do not think the church urges its members to base testimonies on historic events. See here: http://www.mormon.org/mormonorg/eng/basic-beliefs/the-restoration-of-truth/how-can-i-know-this-is-true

3-My vote: sand