The fact that the original letter hasn’t been available for years really doesn’t trouble me much at all...as we (all) have facsimiles of it, and we can know the content. Washington and the founders favored religious tolerance...and so do we—at least everyone but the far Left (and the tiny far right). What is the issue?
There are no known original copies of the Bible either...but that doesn’t prevent us from studying it and keeping its laws.
This is a scholar’s tempest in a teapot, IMHO.
Just curious but, what ‘tiny far right’ are you referring to when you say they have no religious tolerance?