To the extent that attitudes are softened at all toward FC [i.e., “further chances” to get into heaven after death], it seems to be largely through the influence of Lewis’s Divorce, which has been running interference for universalists and their fellow-travelers for years now. Many evangelicals admire Lewis, and that he presents a picture on which there are further chances constitutes for them a reason for giving FC at least some credence–or at least to not react quite so vehemently those who accept, or even just seriously consider, FC.
It’s fun to speculate about what Lewis thought, but I think the obvious main point of The Great Divorce, and the reason for the whole “tour bus from hell spends a day in heaven” conceit, was to make the point that, even if people are given a second chance to get into heaven–even if they get that chance after seeing heaven in all its glory–they’ll choose hell anyway. It’s a parable about the complete lostness of man and how his selfishness blinds him to God. DeRose makes the same point himself:
I’ve always taken Divorce to be mainly about the various tendencies we have–different ones for different people–to “choose against joy.” These are tendencies we (many of us: again, different ones for different people) have now, and constitute a problem for us now, and so are worth exposing. That account of what Lewis was trying to get at provides an account of why he would include the ability to move from hell to heaven as an “imaginative supposal” of his story, even if he wasn’t even speculating as to whether that will be the case in the afterlife.