1st plant is easy to propagate from leaves or stem cuttings. Yes to what you said - clip, callus and then stick into a fast draining mix. This one doesn't really need much callusing - stem is thin. They have tendency to grow long stalks, so lots of light is needed. It should color-up more red in enough light. Leaves fall off very easily - just the way plant is.

Your other leaves are starting to grow roots. They do not need watering for a long time: there is plenty of moisture in mother leaf. That is probably why other leaves rotted. It is best just to leave them to grow, without pulling out of soil and disturbing fine roots. Plantlets should start growing too. It may take a while, and some plants are faster than others.

Before beheading plant, perhaps you should repot (it looks like in original soil?). When you take it out, maybe there is more than 1 plant and could be divided. Not sure why you want to behead it? - it looks still quite compact.

Thanks, I'll get that first one repotted then.

I wanted a second plant to go into a smaller pot, but if it's not the right time for it I'll leave it. I didn't re-pot, it's in some kind of very coarse coconut hair, no soil. But it seems to be doing OK so I left it alone...?

I do not grow succulents in coir so have no suggestion. I do not use peat or cocopeat in mixes, there are some ppl that do. It may or may not retain water - in very hot climates that could be OK. Looks like your plants are indoors, and lots depends on how you keep them indoors (light, temps, watering...)