Re: Sims not staying dead
I have experienced this exact same problem with multiple sims.
I have found that if they die from an emotional reason (i.e. embarrassment, anger, playful), then the reaper will come, collect the soul, leave the urn/gravestone, and then the sim will be alive and well as an NPC "townie".
I also noted that one of my sims that the happened to was invited over to my sim's house. When conversing with the alive/dead sim, when looking at the description and relationship between the two, it said that the (now) NPC sim was my (current) sim's "deceased wife".
Dying of old age does not appear to cause the problem, and I have not seen it happen with any other types of death (i.e. temperature).
I use MCCC, but have tried it without MCCC, and the problem remains.