First, about whether Lewis's portrayal of Aslan is consistent with “the Christ of the Gospels” — I would say both (a) Yes and (b) That depends. Taking the Christian scriptures as a whole, we get three interventions into this created order by the Second Person of the Trinity. First, we are told at the beginning of John's Gospel that “all things were made through him” and, similarly, in Colossians 1 that “by him all things were created”; for this, see The Magician's Nephew. Second, the Incarnate Christ is the one we see in the Gospels; for this, see The Lion, the Witch, and the Wardrobe. And then there's the Christ who “will come again in glory to judge the living and the dead,” as the Creed says, and whose role as Warrior and Judge is described most fully in Revelation; that's The Last Battle. Traditional Christian theology would say, and therefore CSL would say, that the character of the Second Person of the Trinity is utterly consistent but he plays different roles at different points in history.
Now, about Susan. The old Problem of Susan. Adam writes, “ I do not claim that Susan is forever banished from heaven; I say that at the end of the Narnia books she is excluded, and so she is.” I genuinely don't think this is the right way of putting it, because it gives the agency to someone other than Susan — which might be okay if Lewis were a TULIP-affirming Calvinist, but he wasn't. I think that the right way to put it is to say that Susan simply chooses not to return to Narnia. That we paltry little humans have the power to refuse God is a point that Lewis returns to often in his theological writings. As he writes in The Problem of Pain, if we demand that God leave us alone, “that is what he does” — and, interestingly, Lewis prefaces that statement with an “Alas,” as though he might well prefer God to operate in another way. (Which also helps us understand that in sparing Susan from the train wreck that kills the rest of her family he is trying to give her a chance to turn back around towards Narnia. However, the emotional tenor of all this is muddled by this catastrophic contrivance to get the rest of the Pevensies into Narnia one last time; it's one of Lewis's unwisest narrative choices.)
I think this point — that we can refuse God and that some of us do — was important enough to Lewis that he was determined to get it into the Narnia books, but how was he to do it? The point wouldn't be made strongly enough if any of the less dominant characters embodied it, so it had to be one of the Pevensies. He couldn't make Lucy a backslider: she was the one who had always had the greatest faith and the greatest spiritual discernment. And he couldn't use Edmund either, since any renunciation of Aslan by Edmund would destroy the whole portrayal of Edmund's redemption in the first book. So it had to be either Peter or Susan, and I suspect that Lewis was not quite ready to face the possible theological implications of the High King of Narnia becoming a rebel against Aslan. So Susan it had to be. Lewis was backed into a structural corner, as it were.
This is not to say that Lewis didn't have some deeply troubling ideas about women, only that I think he couldn't have gone in another direction if he were going to make this theological point about our ability to be “successful rebels to the end.”
Finally, I want to look at this passage:
My real criticism of this novel relates to a different matter. It is that it ends just when it is getting interesting. The Pevensie kids become the kings and queens of Narnia: King Peter the Magnificent, Queen Susan the Gentle, King Edmund the Just and Queen Lucy the Valiant. They grow to adulthood in this world, until, many years later, they chance upon the lamppost again, and tumble back into our world, no longer adults, now children. Only a few hours have passed on Earth, for all the years (decades?) they spent in Narnia. Then Lewis stops; but this is where the story starts, surely—what would it be like to have an adult consciousness inside the body of a child? To have passed through puberty, and then suddenly to have the hormone tap switched off? You could hardly go back to you former existence; but neither could you expect to live as an adult. Would you go mad, or use your beyond-your-seeming-years wisdom to some purpose? How would you cope? Would you try to explain? Would you betray yourself, and reveal the Narnia portal to the world—would governments attempt to exploit it? The psychological interest in the story begins at the end; but that's exactly the place where Lewis drops the bar down and ends things. Grrr!
I want to look at this passage because I have wondered about this point too, and I suspect that Lewis never really thought this through. In this particular sense the Narnia books are connected in my mind with the old Tom Hanks movie Big, which enacts an especially intriguing wish-fulfillment dream: that it's possible for a kid to assume an adult body and have adult experiences, including living on his own, having sex, and working for a living — and then turn his back on all that and go back to live as a child with his parents. But then what would puberty have been like for him? There's a reason why Blake's Songs of Innocence and Experience can't be read in reverse: “The Lamb” necessarily comes before “The Tyger” and always will.
It's interesting that Lewis to some degree addresses this issue when going the other way: when the adult Kings and Queens of Narnia ride back into the forest where the Lamppost is they have clearly forgotten who they once were and only gradually recollect their previous lives as ordinary children; but whenever they come back into Narnia they seem to have perfect recall of everything they had done there before. Lewis might have done better to reverse the memorial polarity.