C.S. Lewis's Annihilationism

In C.S. Lewis's Great Divorce (as well as other writings) Lewis seems to entertain a kind of annihilationism.  But the occupation of hell and the sort of annihilation which occurs, according to Lewis is voluntary:

There are only two kinds of people – those who say to God, ‘Thy will be done’ to God or those to whom God in the end says, ‘Thy will be done.’ All that are in Hell choose it… No soul that seriously and constantly desires joy will ever miss it.’

As Lewis sees things (or at least as the "narrator" of The Great Divorce sees things), hell is a place of punishment where people ultimately choose to go.  And out of God's grace, God allows people to continue to do their own will after they die.  The punishment is a natural result of refusing to submit one's own will to the will of God. The illustrations he presents are meant to show that doing our own will apart from God's leads to a road of destruction.  And it is we who willfully bring that destruction upon ourselves.

The road to destruction comes in degrees.  Lewis, it would seem, has in mind man as a "rational animal"--humans are animals with a capacity of reason having both an intellect and a will to choose among various ends.  But the rational element is something that can not only gradually be lost but completely lost, leaving behind the mere animal.  The following illustration of a lady who "grumbles" captures the view nicely:

     "Aye, but ye misunderstand me. The question is whether she is a grumbler, or only a grumble. If there is a real woman-even the least trace of one-still there inside the grumbling, it can be brought to life again. If there's one wee spark under all those ashes, we'll blow it till the whole pile is red and clear. But if there's nothing but ashes we'll not go on blowing them in our own eyes forever. They must be swept up."
     "But how can there be a grumble without a grumbler?"
     "The whole difficulty of understanding Hell is that the thing to be understood is so nearly Nothing. But ye'll have had experiences . . . it begins with a grumbling mood, and yourself still distinct from it: perhaps criticising it. And yourself, in a dark hour, may will that mood, embrace it. Ye can repent and come out of it again. But there may come a day when you can do that no longer. Then there will be no you left to criticise the mood, nor even to enjoy it, but just the grumble itself going on forever like a machine." 

On this account there is the person (let's call her), Nancy, and through habitual grumbling, grumbling becomes her sole pursuit.  Through years of succumbing to her desire to grumble, her one and only end becomes to grumble.  But then with only one end (Lewis seems to be suggesting), there is nothing else to choose and all she can do is fulfill her animalistic impulse to grumble.  

Thus spoke Lewis.  But what exactly are we to make of Nancy?  I'll assume Nancy was once a person and that the animal that remains is not a person.  So Nancy was a person, and a person presumably has an ability to choose to act on desires other than merely animalistic ones.  Did she cease to exist or is she now a mere animal?  Lewis seems to leave us with the following options w/respect to what we should think of Nancy:
1. Nancy ceases to be and all that remains is an animal not identical to Nancy or 
2. Nancy (the once rational- animal) becomes a mere animal.  

This raises numerous questions.  Here are six:  (a) Is a change of this sort possible? (b) Even if it is possible for this kind of change to occur (say, by God), do I have the kind of nature such that it's possible that I could annihilate myself?  (c) If this is possible, would God allow it?  (d) Is Lewis committing himself to the view that animals are basically machines?  (e) Why would the "grumble itself going on forever like a machine" not be "mercy-killed"?  Why would it go on forever?  (f) Is it the nature of a (mere) animal to live forever (Aquinas thought not) or does it take a special miracle for it to live forever?

Here I will briefly pursue only the first question and will not come down on a definitive answer.

Adopting 2 has the following consequence: If a human is a rational-animal, then Nancy would not be essentially human, for Nancy could become an animal with no capacity for reason or free choice.  We will thus have to adopt one of the following neither of which is intuitively plausible:
3. One can have a human nature while not having any capacity for rationality 
4. Kinds of things such as I am are not essentially human 

Adopting 1, though, also has a theoretical cost, for we will have to adopt one of the following:
5. Nancy was never a living organism (and thus not an animal) in the first place, for Nancy ceases to be but the organism remains (she was a person with psychological properties not identical with all of the properties of the organism) .
6. Nancy was an organism but one which was once co-located with the animal organism that continues to exist after she ceases to be. 

Out of those possibilities 3 and 5 seem to me more plausible than 4 and 6.  But perhaps the best option would be to reject the view of annihilationism that Lewis was entertaining.  Or perhaps there are other options I haven't pondered.