This all seems pretty reasonable and pretty likely to me! The Mormons would likely use all kinds of captured US equipment and a hodgepodge of smuggled Confederate arms spliced with more homemade guns and bombs. The more modern US equipment would likely be looted off the battlefield and from soldiers as needed. Older equipment would make up the vast majority of the weapons the Mormons would have. I think the only iffy thing about this would be the Mormons having a makeshift "air force" or "bomber force". Granted, you choose outdated civilian biplanes and and converted crop-dusters, with the total number only reaching to just 10 total aircraft. I think an explanation as to how exactly they come to get them wold be nice. So its not like the Mormons are ever able to field a force capable of ever squaring off against the US Air Force in the skies, which is a good move. I'd say these planes would be used strictly for night attacks to harass the US troops on the ground.