This looks very complicated for me.
LibRaw API is file-oriented. One pass filename to LibRaw::open_file(), then operates with LibRaw object which hide all internals into it.
For .MLV files there are many complex entries:
- .idx file that contains fileIndex (but not file names)
- .mlv (?) files whitch contains fileIndex and fileCount.

From LibRaw API view, it's looks like we need to pass filename.idx to open_file(), but how to recover .mlv filenames from .idx? Is LibRaw need to read headers from all .mlv files from folder or what? What should it do if some files are missing? What should it do if several files with same fileIndex exists?

LibRaw (single-image oriented) API is simple:

LibRaw lr;
lr.imgdata.params.shot_select = N; // by default it is 0, generally shot_select=1 is used after 0th frame read to merge two sub-frames for Fuji cameras; that's all
lr.open_file(char *filename); // Opens file and reads all metadata
lr.unpack(); // unpack RAW data
lr.dcraw_process(); // to get RGB bitmap from raw

This is definitely not movie-oriented way. For movies one need other API.

BlackMagic is another story because separate DNG for each frame are stored. RedOne support is not used anywhere AFAIK.