Alicornutopia

Multiple cohabitors causes any problems besides the time sharing the body?

It all boils down to time-sharing, but it would also be difficult or impossible to arrange for a specific person to wake up when you went to sleep (if Alabaster, Bird, and Candle all live together, then sheer coincidence could have Alabaster and Bird swapping off without Candle waking up at all for arbitrarily long periods of time).

I would have thought whoever was least tired would wake up, or at least be more likely to do so. Wouldn't Candle at least get to wake up if both Alabaster and Bird are mentally tired?

Can anybody not born cohabiting learn to fade out instead of sleeping, at least once they learn some people can do it?

Barring people born already together like Holly and Crystal and Book, you can only fall asleep if you are sufficiently tired. (Book is now subject to standard restrictions on when he can fall asleep since moving in with Lightning.) This leaves lots of leeway, but it does mean that by the time Alabaster can fall asleep, neither Bird nor Candle are significantly tired. There are amounts-of-tired, but they're crossed through on a shorter timescale than it would take Alabaster to fall asleep. Most people cannot learn to fade back instead of sleeping.

What would happen if another person was plopped into Holly-and-Crystal?

Goooood question. I could see it working either way - they become like Holly-and-Crystal and automatically gain the ability to fade instead of sleeping, or they don't, and Holly and Crystal's inability to fall totally asleep and force the new cohabitor forward leaves new cohabitor totally asleep all the time.

At which point presumably Holly and Crystal qould experience guilt and try to learn to truesleep (as Book managed to). If that failed, woukd they try to transfer out? That wouldn't guarantee the other person waking, but would give her the best chance, right?

Holly and Crystal could try to learn to truesleep, but I don't see them managing it. And they'd be extremely reluctant to move out and lose their closeness to each other at considerable risk to themselves in case they can't truesleep even moved-out, especially when this would also mean giving up their body to probable death for both the body and the newbie. I'd expect them to try to shove the new person forward in the hopes that eventually they'd figure out how to front rather than doing anything drastic.

Why would this be risky for them? I think I'm missing part of the transfer process. I also thought the “might not wake up” still left a greater chance of waking than you're implying.

When Book moved out, he became subject to normal-person rules about sleeping. But Book was always, of the three, able to fade back farthest. Crystal can do it less and Holly much less. If moving them affected the wrong things, they might find themselves unable to sleep nonphysically after being dislodged from their original body. (This was a risk for Book, too, but they tried it.)

I don't have exact probability in mind for the odds that someone left sleeping alone in a body will wake.