Lewis himself actually articulated your believability argument better than anyone else I've ever read. But all I can do is paraphrase: it's ok for an imaginary world to have talking animals; they simply must talk as if they would if they really talked; in other words, an imaginary world must be consistent with its own rules.
Which is where you miss it, because you are missing that Narnia is an imaginary children's world, not an imaginary adult's world.
In Braveheart, we wouldn't expect anybody to win a swordfight except by training, because, well, it is still our universe, though a different time.
But in Middle Earth, for example, did you happen to notice that hobbits killed huge goblins with little daggers? That a lady killed a witch-king who had decimated entire armies of male warriors -- because some prophecy or other said "no man" could kill him?
In an imaginary children's world, hobbits and ladies and, yes, boys actually do win swordfights with goblins, because of their courage and their goodness. Unbelievable, you say? then it's been too long since you were a boy.
You missed everything important about every character. Lucy's character was based not on her "cuteness", and not just on her age, but on her goodness. The principle being that those who are good perceive a different world than those who are bad. You notice she always perceived what the others missed: Narnia, the fauns in the fire, the dryad, Aslan leaving the camp, Aslan leaving the world.
The point of Susan's character was not her tendency to kill fun; it was her reductionist logic, which chopped off entire chunks of the cosmos for her.
The point of Peter's character was the question of whether or not he would be willing to take his father's place and take responsibility for the others -- to the point of self-sacrifice. He did do that, of course.
And so on. You missed everything. Only boards in the back of your wardrobe.
If they are going to make a movie into a book, and much more so it it's from a series, the producers and director must do it such a way as those who haven't read the books will understand and accept the plot, as well as bond with the main characters.
All I'm saying is this movie didn't do that.
"Only boards in the back of your wardrobe."
Good one! I know people for whom this is the perfect description.
Ewoyn was a Shieldmaiden of Rohan. She was a trained fighter, not just "a lady" .
And she would have lost a fair fight with the witch-king. She only won because he was stabbed in the back by a hobbit.
"I would have succeeded to, but for those meddling hobbits"
Hollywood version...
The book, from memory (paraphrase, Tolkien does it better):
Thus passed the sword of the Barrow-downs, work of Westernesse. But glad would have have been to know its fate was he who wrought it long ago in the North Kingdom when the Dunedain were young, and chief among its foes were the dread realm of Angmar and its sorcerer-king. No other blade, though mightier hands had wielded it, would have dealt that enemy such a blow,[not sure of that phrase, there], cleaving the undead flesh, breaking the spell that knit his unseen sinews to his will
So the power was in the sword, not Eowyn.
Cheers!