When a magical girl falls into despair, her soul gem turns into a grief seed, which makes her a witch.
Sounds about right. Fun times all round! Good old 'Dokes.
What does "projections of the magical girl's heart" mean?
Madoka is quite meta in it's Magic sometimes. Consider Wishes, which just break all laws ever, by Kyuubey's word. The exact mechanics beetween Witches and Familiars aren't touched on much beyond they're some kind of life cycle, but we can infer some things.
"Projections of the magical girl's heart before she became a witch." Since that's a one-way trip, witches would seem to have a finite pool of experience to base Familiars on.
Witches maintain barriers to protect themselves and entrap victims.
Familiars are a division of a Witch's essence who, after consuming several humans will grow into a Witch with their own barrier.
Witches seem to be creatures of emotion, given that they're born from grief, and their very appearance is a complex cocktail of the Girl's emotions, desires and fears. This makes the line about "projections of the heart" make more sense.
I imagine the process to be something like this.
- Magical Girl becomes a Witch. Her appearance and powers are defined by her experiences prior.
- Witch feels one of her remembered emotions in her essence
- The emotion is so strong it is "divided out of the essence" and becomes it's own, autonomous being with it's own essence, based on that emotion.
- Feeding on humans develops the familiar into a Witch somehow, at which point it can erect it's own barrier and become truly autonomous.
And an example:
- Charlotte is a Witch. Her Magical Girl self loved cheese.
- Charlotte wants cheese.
- Charlotte really wants cheese.
- Charlotte wants cheese so hard, that a little bit of "WANT CHEESE" pushes out of "her", subdivides from her essence and drops off.
- Naturally it's entirely "WANT CHEESE" and is thus single-minded and weak.
- It consumes a person. This somehow grows it's essence a little. It's a little smarter and stronger.
- Repeat this until it's grown enough to handle a barrier. At this point, it can leave the vicinity of it's "parent" safely and can protect itself as a fully grown Witch.
Although, this does raise the question: Does every familiar bother hunting and killing people? How fast is their growth rate? The Witches in the series never seem to handle more than 10-20 people at once, maximum, so it may not be that viable, considering the Witch must need feeding too. Food for thought. (haha)
Does it mean a magical girl can become more than one witch?
By the logic we've been presented with, I can see no reason why not. Though, there's no mention of whether a Familiar grows into it's parent Witch, or something else. It does raise questions about "Witch Farming" as a viable tactic though.
Kyouko's "Wait around until they become Witches" seems a very passive route. Why not directly feed your Familiars humans if you really don't care? Looks like the goodness of a girl's heart is the only thing keeping us safe. Fitting, non?
From discussion in the comments, it's become clear that we don't really know if Familiar!Witches actually drop Grief Seeds apart from these lines from Kyouko
"You just have to wait until it eats four or five people and becomes a witch. That way, you'll be able to get a Grief Seed from it. Why would you want to slaughter the chicken before it lays any eggs?"
Yes, Kyouko thinks they do, but we never actually see it happen. Thus, it makes the next section only relevant if they do. If they don't, everything's fine and there's no imbalance in the first place, so potato-patato.
Also, how does this affect the universal entropy?
The same way the rest of the series does, this doesn't necessarily change anything.
Initially thinking about this, yes, I got a little freaked if you could potentially get fifteen witches where you had one, and that'd definitely throw something somewhere down the line off.
But when you get down to it, there are a few points of information we can use.
Magical Girls have been around for a looooong time. Consider the historical ones we see for sure. In fact, Kyuubey flat out states we'd still be in caves without his kind, so that kind of dates them. Though, if you trust his word is another matter entirely.
Only with "The Most Powerful Witch" will Incubators have offset Entropy for any reasonable amount, and she was ludicrously powerful. I won't reason about it here for plot's sake but she's a complete fluke is my point.
So, threading this together.
If Magical Girls have been around for a long time and only now, with this completely, universally, OP Witch, will we actually make a dent, we can infer that the Entropy paid back for a "normal" witch isn't actually that much. Kyuubey can harp on about his "tremendous amount of energy" all he wants, but from the looks of things they aren't really doing a whole lot.
The way Kyuubey treats the girls, even apart from his alien detachedness, would suggest that they're disposable to him. And they are, because this entire scheme is ultra-long-term. He has a quota to reach, yes. But his deadline is literally the death of the Universe. A girl here or there, a few witches, it doesn't matter. Thousands, millions, now they might matter. But the impact of the average individual Girl or Witch doesn't seem to. Thank you for making me logic out this horrifying new revelation, I really enjoyed it. :)
- Normal Witches are worthless in the grand scheme of things, since we've been fighting them for years and Kyuubey makes no mention of progress.
- Entropy would need many multiples of the time Magical Girls have been around to even begin to be repaid.
- So you can have ten, fifteen witches from one girl and it's still chump change.
And yes, this means there are ten, fifteen Witches in the locality now. But all Kyuubey has to do to rebalance the teams is lie to three, four little girls. And we all know how good he is at that!