Can the 45LC Remmie conversion be used as cap'n'bal? Possibly.
The first samples made by Uberti had both cylinders, and some articles were written and ads produced indicating that it wouild be a convertible. However, when they went into production, Uberti did not fit a percussion cylinder so they were cartridge only. The cylinders and frames of the conversions are slightly bigger than the cap'n'ball '58's in order to be able to produce it in a six-shot 45LC version without tapering the cylinder holes, so a standard '58 percussion cylinder will not fit. Uberti did produce some of the larger sized percussion cylinders and Cimarron has them on the shelf, but they have not been fit to guns. You can order one of these percussion cylinders but it will have to be fit to the gun which I understand is difficult to do and have the gun work well as both cartridge and percussion.
I imagine it was a decision based on cost to not sell the guns with both cylinders, since the extra cylinder and the time to fit it would have raised the price and many purchasers wouldn't particularly want to shoot it as percussion.