Is anyone else tired of the 'problem with susan'
One thing that puzzles me is that, whenever Aslan tells a character from our world that they will not return to Narnia, he also says that he exists in our world and has another name - but this is never followed up. I think it was quite deliberate that Lewis did not spell things out with no room for interpretation, otherwise he would have been accused (quite rightly) that he was using his stories to preach. But there's no indication in what the Seven Friends of Narnia say that when they got together to talk about Narnia, Aslan-With-Another-Name was with them. What if Susan had been with them to begin with and was expecting Aslan-With-Another-Name to be there, and when he wasn't, that fuelled the disenchantment and doubt?
That's an interesting thought, although the end of Voyage is the only time we're given Aslan's exact words to those who wouldn't be returning to Narnia — when he tells Peter and Susan at the end of Prince Caspian, it happens "offstage" and there's no indication of what Aslan actually said. We're only given Peter's account of it afterwards, and when Lucy asks him if he can bear it, he just responds "Well, I think I can... It's all rather different from what I thought. You'll understand when it comes to your last time."
We can definitely assume that Aslan did tell Peter and Susan they would need to learn to know him by his name in their own world (although first-time readers of PC who hadn't yet read VDT wouldn't know that) and Peter is trying to fathom what that means — and from the fact that he's so calm about it and tells Lucy she'll "understand" some day too, I'm wondering if he's already at least starting to realise who Aslan is. But we're never told at all how Susan took that last conversation with Aslan or what her response was, so that might be significant. The implication could be that unlike Peter, she hasn't grasped anything of what Aslan meant and this is a further hint that she's already starting to pull away from Narnia and its influence on her.
I don't think we're ever told in detail what the Seven Friends of Narnia said or did at their meetings (it's a while since I last read The Last Battle), but I wouldn't imagine them as being like a private religious meeting where they would expect Aslan/Jesus to be literally with them in person. Lewis was a fairly conservative Anglican and I'm guessing his idea would be that his young characters, once they'd realised who Aslan is in our world, would become regular churchgoers as Lewis himself became after his conversion. But as you say, he deliberately doesn't spell it out and risk sounding preachy.
I guess in the end it's also left for us to imagine what exactly put Susan off Narnia, or rather led to her convincing herself that this other world (where she had actually reigned for years as a Queen!) was only a silly childhood game. I don't get the impression it was any one thing that triggered it, but rather a gradual process that we can already see beginning in the earlier books. But as Lewis himself wrote to a young reader, "there is still plenty of time for her to mend, and perhaps she will get to Aslan's country in the end — in her own way." Which I've always felt she did do, at last.
"Now you are a lioness," said Aslan. "And now all Narnia will be renewed."