I tried to scan 2 of the slides today. I checked for a slide setting on my printer (a Canon) but didn't see anything. The darker slide actually turned out better than the lighter slide, but neither was good. I think 2 of the slides are of my sister when she was about a year old. I will definitely get those converted for her birthday this fall.
It sounds like you are working on a giant project. My brother-in-law has been scanning family pictures and documents for many years. He added some to genealogy sites. It's always a treat when he shares pictures. I have a friend who was left the lifetime work of a WV news photographer, who was his cousin. Since my friend retired, he scans pictures whenever he can. The work stretches from the mid 50s until about 2000. My friend has been adding to the work by taking pictures of local buildings and streets when he is out driving. I recently joined a Memories group on Facebook. The owner is always asking for vintage pictures of Woodsfield/Monroe County Ohio. There is a lack of vintage pictures of buildings and places for sure. I think when we used film and cameras, for a lot of people, it was expensive, so they took pictures of loved ones, and not their town. At least in rural areas in SE Oh.
I have a couple of milk-glass positive photos (opalotypes) but they aren't of family members. When I did research to find out about them, I learned that there is a lot of interest in taking photographs using the very early methods. Do you think you'll ever have the ultimate scans, the way that technology continues to improve?