My overall question is, in a old dog of an 1873 trapdoor, would it be safe/advisable to load the above mentioned 54 grains of IMR 4350. If not, what would you suggest?
In an original 1873 Trapdoor,
I would not use IMR 4350 or any other smokeless powder. Wouldn't hesitate to do so in a reproduction Trapdoor, but not a nearly 140 year old rifle. Real BP (or a sub such as Pyrodex) would be a lot safer, and more fun, in an old action.
That's just my opinion, without seeing the gun . . . If you trust the gunsmith, load some up and let him pull the trigger.