My TBM sister has a son (XXXX) who completed medical school in 2009 and is now a practicing doctor. I have considered having the following discussion with her.

When XXXX entered medical school, what was the timeliness of the training he received? Obviously it was the most recent medical knowledge available.

What if the medical school had told him that they would teach him how to be a doctor based on the medical knowledge that was available in 1840? Or maybe the best medical knowledge available in 1940?

Obviously that is a totally stupid question. There is no way that you would want XXXX to be taught medical science based on the best information that was available in 1840 or 1940. You would expect that only the most up-to-date medical knowedge would be used in his training.

Now let's change the topic of discussion to religion. As faithful members of the Mormon Church, you and The Church taught religion to XXXX based on information from the 1840s.

Thanks to the Internet there is abundant information about The Church that has come to light in the last few years. Why not teach XXXX about The Church based on the best information from current years? Why is it logical to only draw on Church information from around 1840? Can you answer those two questions?

I think I follow you, but not sure this will make the point you want.

For example:

The First Vision had yet to become part of Church doctrine and history--they had yet to develop the story.

Polygamy was being practiced, but the Church actively lied about it being practiced up until it was publicly announced behind the Rocky Mountains of Utah in 1852.

Mormonism has been a fraud since founded in 1830--it has never been an honest enterprise.

If I'm following your thinking correctly, I think this might backfire.

Actually, the church doesn't like to teach a lot of its past, only the faith promoting bits. They like to sweep a lot church history under the rug because much of it isn't very flattering. The church would much prefer that you mostly focus on its current positive teachings and forget about everything else.