I've been thinking about this one for a while, and I don't know that I've come to a satisfactory conclusion. But I have decided to post my thoughts, in hopes I'll provoke further discussion.

As Ajnos points out, the Lady of the Green Kirtle follows a round about way of taking over the country, since she could have just conquered Narnia without Rillian. While this is speculation, I think she possessed a jealousy toward the legitimate queen of Narnia (as well as a desire to take over the country). I think she wanted to be the true queen of Narnia, rather than just a conqueror.

While part of why she killed the Narnian queen was to get to Rillian, I believe she was also motivated by envy toward the queen. After murdering the queen, she kidnapped and enchanted the prince because by reigning as his consort, she would fulfill her desire to be the true queen of Narnia rather than a mere usurper.

She may also have feared Aslan, and known that since he chose Caspian as the king of Narnia, she might be protected from his wrath by being tied to Caspian's son.

I am still thinking through how the silver chair worked, and why Lewis chose to express Rillian’s enslavement through it.

Thanks, Ajnos, for a thought provoking topic!

I like that idea, Yorick, that somehow she was grasping for legitimacy.

Speaking of the silver chair, just recently, I heard something that made me rethink how I saw it.
This song by the Gray Havens "called Silver" is inspired by Lewis and Tolkien's conversation where Lewis said That he liked myths but they were lies breathed through silver and Tolkien said that Christianity was the True Myth. The song makes me think that Lewis could have used the idea of silver, if not to represent lies themselves, at least to represent the pretty package that lies can come in. Like how a "Silver tongue" is someone who is very eloquent and persuasive.
The song even has a line where the true myth "Broke the spell that tied us to our silver chairs"

The Gray Havens explain and sing the song here:
http://youtu.be/27JZVXR6oAs?t=19m46s
(This is on their own youtube channel so no copyright infringement to view it)
(Also they have made the album available for free download here: http://noisetrade.com/thegrayhavens/where-eyes-dont-go )

Nice thinking, Yorick. I've considered that too, but not to the extent you did, so I've still got things to think about. 😀

Hm, that's interesting, Swan, about silver being lies. It seems to make more sense that how I've always tried to think of it (though I'm not giving up on my thoughts just yet!).

Silver is symbolic of redemption (pardon me while I suddenly and for the first time ascribe significance to Edmund's silver crown...), so I've always wondered exactly how this instrument of evil could possibly tie into that. Now of course, just because something is used doesn't mean that it was used for its symbolism.
The whole silver = redemption thing kinda ties into theories about the chair's use. (btw, I'm sure I'm not the only one who's ever wondered why LotGK couldn't/didn't keep him in a permanent state of enchantedness.) Obviously, his right mind is restored during his sessions in the chair, but unless LotGK is freakishly sadistic and deems it the best way to torture him (why? no idea, but considering that this theory that involves an extremely sadistic witch...), then I doubt that was exactly part of her plan.
Perhaps both lies and redemption are at work: she binds him to the lies, Aslan uses it as a catalyst for redemption (ie: he's in his right mind and cries out in the name of Aslan)?

I had not heard of its being a symbol for redemption, but I do like the idea of it adding to Edmund's story. A quick google search taught me today that silver has been used as a symbol for a multitude of contradicting ideas. I'd naively thought it a much simpler interpretation. Still hearing that Lewis himself referred to "lies breathed through silver" it inclines me to think that he might associate silver the kind of beauty that might disguise a lie as the silver chair in fact does.

I think the chair was in some way renewing her spell over him, because if he got out before his madness returned then he would remain sane. Certainly she did not wish him to have a sane hour at all and I think it must have been the limit that Aslan put on her power.

Those are interesting thoughts about the symbolism of silver.

I guess what confuses me most about what the witch was up to was why she wanted to make the legitimate heir invade his own country. Unless...just thought of this now...she kidnapped him and kept him till Caspian was off on his voyage making sure Rilian would be under her spell when it was time to take the throne. Of course, the problem with this is Caspian isn't dead yet.

Here's what I wrote about the issue during last year's Summer Challenge:

http://www.hedgepickle.blogspot.com/2012/07/the-silver-chair-chapter-11.html

Weapon of terror, maybe? Think about the psychological impact for the kingdom she intended to conquer. The Narnians have been missing their prince and heir for years. Many probably believe him dead. Then he comes back... but it's not him. It's his shell, controlled by someone evil. And the prince you love is attacking and destroying you, but you can't assassinate the conqueror because maybe, just maybe, he's really still himself inside and to kill him would be to kill your last chance of seeing the real Rillian, Caspian's son, the beloved Star-queen's son, again. And on a more intimate, less national scale, if he retained any shred of his own consciousness--even in dreams--after his rise to the surface, think of the implications on his own mind. It's a sadistically clever way of both wreaking national morale, and deconstructing her hostage's self-respect.

I think everyone is right when they say she enchanted Rillian instead of killing him, because she wanted to be viewed as a legitimate successor to the throne. I also think the Lady of the Green Kirtle wanted extreme control over everyone. She wanted to control thoughts. She made the gnomes temporarily forget everything happy in their lives and tried to make Eustace, Jill, and Puddleglum forget about the things that mattered the most to them.

I've also noticed a pattern of pretenders running through the Chronicles: Beaver says the White Witch wants people to think she is a daughter of Eve when she is really the daughter of a giantess and a jinn, Miraz illegitimately acts as king after murdering his brother, the Lady of the Green Kirtle wants to take the role of the queen she murdered, and Shift dresses Puzzle up as Aslan and pretends to be a man.

I think it was Lewis's way of showing that rebellion is not just a desire to disobey God, but a desire to steal the very place of God. Those who rebel against God want to take his authority and make it their own.

Ooh, good insight, Yorick! 😀

Excellent point, Yorick. Evil does that, pretending to be good, and Lewis shows that in a number of ways. There are always excuses, disguises and lies because even the villains feel the reality of right and wrong.

"Even the villains feel the reality of right and wrong."
How many villains do you think saw themselves as the villain of the story?

I doubt most (if any) did. Perhaps Jadis and perhaps LotGK, but, say, Miraz or Pug wouldn't have.

Mm. I guess longer life/greater knowledge might carry a certain amount of moral responsibility with it.

But even with Jadis, I kind of wonder whether she totally embraced the villain-role, or just saw stuff through a warped glass. I mean, in her own world lower-ranking persons don't carry value in her eyes, but she stands on the letter of legality with all of her might when it comes to nobility. She will not use her WMD unless her throne is being contested by a potential usurper who has broken her word as to the rules of the great game. Her ancestor was right to kill his nobles because they were plotting treason. She would not contest Uncle Andrew's local authority were he a ruler with the right blood. She expects and basks in the praise which she thinks is her due. There's a certain logic to it--she really thinks she's all that and a bag of chips, and that everybody else should, you know? Her pride in herself and her magic and her family name seems to have her blinded. And then she gets thrown into a new world where the rules are different and the laws are different and nobody knows her, and there's a power which isn't vulnerable to her and which could squash her like a bug. And she learns how to use new magic, and she tries to carve a position for herself--Emperor's hangman, rumored part-Human--where she can get a measure of the respect, or at least the fear, she's used to and considers her right. At least, that's how I see her.

With LOTGK, I don't know of her background, or what brought her mindset to where she was willing to hide in the deep places and mass an army and use subterfuge for however long in order to be able to target Narnia, in particular. Why did she have a grudge against Narnia? Or what were its resources and attractions for her? Of course she must have known that her plans were cruel, unless Enchanted!Rilian voiced her own mindset and she saw herself as grand and beautiful and pure and thought that bursting up out of the earth with the prince at her side was a big joke and that surely everyone would get over it once they saw how awesome she was, which I rather doubt. :p But I still don't know what was in her past to make it so attractive to her.

I've just remembered a Czech fairy tale...
In very condensed form and as far as I remember it, a prince enchanted into a white snake marries a princess (in the snake form) and turns back into a human at night, to her immense relief. So when she finds his snake skin, she thinks it's a good way to make sure he stays human forever and burns it; it does work, but the side effect is that he forgets her and leaves her? He's definitely gone. She then eventually finds him "married" to a queen, who's really the witch who enchanted him to begin with (because he did not want to marry her). So she tricks the witch into allowing her to stay with him three consecutive nights; but the witch gives him a sleeping potion. But he begins to be suspicious, so the third night he doesn't drink it, and finds his wife there and they live happily ever after.

So. Obviously it's by no means the same story, but I've realised it features some similar motifs (snake-shaping people, a witch who tricks a prince into marrying her, and the enchanted prince losing his memories...), and it might be interesting to see if Lewis may have known similar tales, and taken those motifs and turned them upside down and see what that says about his motives in writing the book the way he did.