Have you tried this yet?:
Apple Support: Install the latest compatible version of an app on an earlier version of iOS or OS X
If that doesn't work, you could join the LDS Tools for iOS project here on LDSTech, and try downloading one of the old betas, like
1644 or earlier.
For others who might be reading: Note that the Apple process for installing an older version of an app doesn't work for installing Gospel Library 2.5, because Gospel Library 3.0 was submitted as a separate app so that the Church could distribute both of them for a time. But it might work for LDS Tools or other apps.