Looks like there's a drainage problem and or water retaining soil. I would suggest mixing in 50% perlite or pumice. This will add extra drainage to your mix. Most readily available cactus soils are loaded with peat. Peat just isn't good for succulents in large amounts. So if you mix 50%-60% perlite or pumice in the mix it breaks it up, adds drainage, and aerates the soil. Peat either retains moisture or once dried out turns to concrete and repels water. Also chokes the roots when dried out. So you will either have a soggy mess or a hard mass. So the perlite/pumice prevents peat from what it naturally wants to do.

When repotting always make sure to clean the old soil off the roots:).

That plant may need more light than the others. Days are shorter and North light is good for painting, not so much for sun loving plants. Maybe supplement with a desk light with a daylight fluorescent?

Those lower leaves look like they're not going to make it. If I'm right, i'd gently pull them to the side and remove them. Although it may not look so great, if you can get it turned around maybe you can behead it in Spring or it'll put out more growth and fill out.

Its hard to say if its under or over watered. If you stick your finger in the edge is it dry? When you replanted it, was the soil moist?

The soil was dry, but when I touched the roots lightly, they completely fell off. The stem was also mushy. I think this one's a goner unfortunately.

Yes, it's toast =(

Yep it's gone, but I would suggest repotting your other succulents into a grittier mix:).