Personally I grow my sans in less perlite and more organics. Your mix might be drying out too fast especially if you had it outside. Now if you were watering a couple of times a week that might work with 50% perlite.

He does seem to dry out reaaally fast. Do you think I should sever the more lively plantlet from the dying one?

yep!

Looks good, now give him a big drink ('til overflow) then then leave alone for a while. Assumes you're in warm climate.

Well....I don't know. I am in SW FL where it's relatively warm almost all the time. My Sans are generally larger ones. I keep them in PLASTIC pots (because Sans so often have rhizomes underground which actually BREAK the pots, and I want to let them break plastic if they break anything). My mix is generally 1/3 C/S soil, 1/3 perlite or pumice, and 1/3 manna pro poultry grit. Sometimes it's 25% of everything including both perlite and pumice. Most of my Sans are in very bright light, and I water anywhere from 10 to 14 days. If you are using terra cotta pots, I think you will need to water more often, maybe every 7 to 10 days.

Your repot looks good with a little more C/S soil. Try not to disturb the plant for the winter. Being yanked in and out of soil mix doesn't help. But -- you do have some good roots there, so I think you will okay till spring with typical "minimum Sans care for the winter." These are resilient plants. They like warmth, some bright light when they can get it, good drainage, roots that aren't cold or wet -- and then to do their own thing. Good luck with your rescued plant. I think he will be okay.

You might want to cut back on the fertilizer. Over fertilized Sans exhibit floppy and soft leaves, which otherwise should be sturdy and erect. Over fertilization is also noticeable by the appearance of white spots on the leaves (excretion of accumulated salts) and sudden yellowing offsets. Accumulated salts aren't visible on your pictures, but the leaves are still very young. So you might want to stop using fertilizer all together untill they are more mature.

The health of current leaves won't restore much when you adjust your regimen. A correctly adjusted watering regimen will only be shown by healthy new offsets in the near future. Keep them in the sill and they will be just fine, it might take a while before you notice though. Good luck!

New new growth spotted inside the new growth!