Have you any facilities in any of the software you've got to read just the first session of the multi-session disk? That would seem the most sensible way to proceed if you can.
I feel your pain. Without wanting to sound too preachy, the important lesson here is not to entrust valuable data to just one disc, particularly an unfinalised multi-session disc.
Even if you eventually wanted to store some kind of composite disc, it would have been wise to make at least two copies of the irreplaceable pictures on CD-R and finalise the discs, ideally without using multi-session at all. A couple of CD-R discs don't cost much, and if you eventually decided you wanted additional data on the discs, you could make new discs and, if you really needed the space, throw these originals away.
An alternative would be to use CD-RW for at least one of the copies for now, so that you could eventually erase and reuse the disc. However, CD-RW media can be rather less robust than CD-R.