The Mrs. Ws transport the children around, and shield them when tessering through the Black Thing. So why is it not their way to actually do something like free Charles Wallace from IT?
Charles Wallace entered willingly into IT. He cannot be forced from IT; to do so would probably destroy his mind.
Given that tessering moves around in time as well as space, why couldn't the Happy Medium see if Meg succeeds in the end?
Because Time is not fixed. Until she actually does succeed it hasn't happened yet. The future is not fixed. As we see in A Swiftly Tilting Planet, neither is the past, actually.
It's just the word choice, really — but the shorted distance between any two points is always a line! It just doesn't necessarily go through our perceived three (four) dimensions.
Yeah, that bugged me, too. A lot of people describe it that way, actually, which is incredibly confusing to someone trying to learn higher dimensional thinking.
Sorry, this is a minor quibble, but technically the shortest distance between two points is a geodesic. Because, for instance, if you are traveling on the surface of a sphere, the shortest distance is a great circle, not a straight line. (Granted, it is still a kind of line, I suppose, but then, so is a point, which arguably would be what you get when you fold two space points together—just a one-D line.)
Something's always bothered me about space-folding in general. Okay, you take points A and B, fold the space between them, and step right over the fold to teleport somewhere. So far so good. But what happens to the region that got folded? In relativity models that treat spacetime like a surface (the same way the space-folding explanation the Mrs W's give does), gravity is treated as a dip in the surface and in this case, the fold is a "dip" that's completely pinched off from the rest of the surface. In other words, a black hole. So, in short, wouldn't folding space to temporarily connect two points completely destroy everything between them? Even when it unfolds again, it'll leave behind a smashed wreck: from the universe's perspective, the entire region between A and B suddenly turned into the interior of a black hole, and then it suddenly turned back to normal again. Does it happen too fast to cause damage, or does the math behind what's happening break away from the flat-surface analogy at some point? Space folding is practically a sci-fi trope of its own, but I've never seen a story deal with that implication of the paper sheet model.
Just because two concepts are described with similar analogies doesn't mean they're the same thing. In this case, space is folded through an extra dimension, in much the same way that a flat piece of paper can be folded through the third dimension without altering any two dimensional drawings on the paper. Einsteinian models of gravity that show flat spacetime being dented are simplifications, as mentioned in this xkcd strip. They do not represent folding through extra dimensions.
It also may be worth noting that, even though the book reads like science fiction, there's also significant fantasy elements to it. It could be that tessering is somehow connected to them in such a way so that they either couldn't be explained or else wouldn't be comprehended. That is, the science of the situation may be subject to more powerful things that seem magical.
Why didn't A Wind At The Door reference anything that happened in A Wrinkle in Time at all? No mention of the Mrs. Ws, the fact that they travelled via tesseract, the years that the father was missing, etc.? I mean, I suppose you're not just gonna randomly go "Hey Dad, remember that time you were kidnapped by an evil floating brain?", but still, it felt a bit eerie that the events of the previous book weren't even mentioned, making it not feel like a sequel at all.
I actually read Wind before Wrinkle, and was surprised to find out what order they occur in. There is almost no serial coherence whatsoever between the books; only some exists between Wind and Planet. Yeah, it bugs me too.
Where would a two-dimensional planet exist? Surely not in our three-dimensional universe. (If it did, what kind of topology would space follow going from ordinary space to the two-dimensional planet.)