There's something not to forget in all the ex post facto psychoanalyzing of Lewis - he may not have known what happened to Susan. In fact, he may not have even known that she wouldn't make it beyond the Stable Door until he actually got there in the storyline.
Speaking as an author myself, and having read accounts by other authors such as Lewis and Tolkien, as well as spoken with author colleagues, I can attest to the fact that stories can take on a life of their own. They can take unexpected turns, new characters you didn't anticipate can appear, or characters can do things "on their own".
There's no way of knowing, because Lewis left no record, but it wouldn't surprise me a bit if he was just writing the story and, when he came to Tirian appearing at the banquet of the Friends of Narnia, "discovered" there were only seven attendees. If it worked that way, and if Lewis wrote like I do, he would have asked himself, "Oh - where's Susan?", and his imagination would have told him.
Again, I'm not claiming that this is what happened, just that this kind of thing happens when I write, and apparently it happens to others as well. But if it did work like that, it would leave totally open the question of what ultimately happened to her, because it wouldn't be like Lewis knew himself. Perhaps if he'd picked up a pen with the intent to write that part, his imagination would have told him. I doubt it would have been some kind of dark ending like the destruction of Belbury in That Hideous Strength, but there's no telling. I think if we wanted a clue, we could look to Orual in Till We Have Faces, who did a much more terrible thing than Susan ever did, but found redemption in the end.