I agree that she is not as formidable as Jadis, at least in personality. Her spell did keep Prince Rilian under her control, but she was unable to beguile Puddleglum and thus was unable to sieze control of Eustuce and Jill. This indicates that Rilian was quite subseptable to the power of magic and one could argue that he is not suited to rule Narnia despite that he renders her in two with his blade. Rilian is fortunate he never encountered Jadis, he would have been done for.
Before you point the finger at Rilian, do remember that he was at a much more vulnerable state when he was captured than Eustace, Jill and Puddleglum. The Emerald Witch may not have been as powerful as the White Witch but she was certainly cunning.
Do remember that Rilian had just lost his mother, the queen of Narnia. He was young and consumed by thoughts of revenge. It is my belief that his grief and hatred are in part what made him susceptible to LotGK.
The Emerald Witch created a situation which she knew would devastate him and then she came to him, not in the form of the snake he was looking for, but in the form of a beautiful woman who was able to take his mind from his heartache. She met him at a time when she was expecting to face him as a possible foe and when his guard was down because he was expecting a snake; not a woman-especially not an enchantress.
The children and Puddleglum had the advantage of knowing Rilian's backstory. They knew he was kidnapped by a woman in a dress as green as poison after searching for a deadly serpent which was as green as poison. They knew that several warriors had been lost in an attempt the find Prince Rilian and thus could gather that some sort of magic was involved. Rilian had no such facts. He had no idea he would be kidnapped. All the clues he had were that the snake was green and the woman's dress was green. Big deal. Trees are green. Grass is green. Green happens. Who would count that as a clue until after the fact?
Pluddleglum and the children went on their mission expecting to find a snake or a beautiful woman. Even they, who had been sent by Aslan to help the prince, who had been given signs by the Lion Himself, managed to pass LotGK and Rilian as the Black Knight on the way to Harfang. And this AFTER being given her description and actually knowing what they were looking for. We could attribute their blindness to magic on the part of the witch or stupidity on their part, perhaps a little bit of both. We may never know.
This isn't to say that Rilian was without fault. I would just like to point out that Rilian's mind was clouded by the circumstances and he did not have all the facts his three rescuers had. By the time he was able to think clearly enough to realize he'd been had it was too late and the enchantment was already too strong for one person to conquer alone.
As for Rilian being "fit to rule Narnia" after being under such a powerful enchantment, Aslan didn't seem to be in the habit of choosing people who were "fit to rule". In the Magician's nephew we have King Frank, the cabbie. Seems like a perfect king
Oh I know, how about four kids who accidently stumbled into the world, one of whom also fell into the hands of a witch? Or what about the young orphan who's fleeing for his life to escape his murderous uncle? Did I mention he's a Telmarine, therefore he's not even supposed to like Aslan or Old Narnians? If one was fit to rule all on his own then all the credit would go to man and not Aslan. Aslan is One to choose the unlikely so that His strength is made perfect in their weakness.
As for the White Witch all who faced her would've been done for if not for Aslan. The only reason anyone in the Chronicles ever defeated a witch was because Aslan was the one who ordained the defeat. Aslan had sent Eaustace, Puddleglum and Jill to rescue Rilian and so it was at that time he was able to be freed and all were able to overcome the witch. Because Aslan had willed it so.
It's not all about the evil we face, nor is it about overcoming the evil within ourselves by our own strength because we're powerful enough to do it on our own. It's all about what Aslan can do when we obey Him and it's about Him helping us defeat our darkness. It's about His strength being made perfect in our weakness. Because of Him we are able to take on difficult tasks, whether we're "fit" do to them or not. It's not about what we're capable of. It's about what He's capable of doing in and through us.