Dutch,

For your Colindian rifle, I would not reject the idea that a round ball, soft or hardened, or a cloth-patched hard or soft round ball might work just fine.

The advent of a successful large calibre and strong enough breech-loader was the only reason round balls were cast aside for large and dangerous game. Until those stronger breech-loaders came along, hardened round balls were preferred as they out-penetrated the soft lead conicals needed for accuracy.

The ball loads I developed shot well enough to 100yards in my SXS smoothbore - bare but slightly undersized, but were held in the middle of the bore by a cupped wad. I used plastic, however a fiber wad or thick card with a hole in the middle, hard card underneath would also do the same work. The wad will be groove diameter at the breech. The ball can be undersized but as it is sealed and held in the middle by a cupped wad, they were still quite accurate.
I did mount a scope on the SXS for load development, but with that, I was able to repeatedly hit a 12" square plate at 100 meters, offhand, right/left - right/left.

I also experimented with cloth patched round balls in a straight rifled Husqvarna SXS in brass ctgs. where they showed better accuracy than the naked lead balls that were larger than the bore.

The cloth patched balls were seated and crimped yet appeared to perform perfectly. I was concerned the case mouth crimp might strip off the patch, but due to the pressures from behind, I assume, that did not happen. They shot very well, and cleanly as they were lubed with a BP lube - Track's Mink Oil.

I do hope this give you HOPE along with some ideas to attempt in order to get this marvelous gun working.

What a challenge - ought to be fun! Beer or Whisky helps cure frustration.