What on earth is The Last Battle going on about, with that donkey and Plato and the poor girl who gets sent to hell for wearing nylons and lipstick?
No poor girl is sent to hell. Even for lipstick and nylons. She is just not set for Heaven yet when they are talking about her in the book. Although, to be fair, I think these things - or rather lipstick according to present standards of modesty - though not mortal sins in themselves may indeed occasion other sins that are mortal ones and that therefore do send one to hell. So, when they are talking about Susan, she is not sent to Hell, but she is heading for it. Not as much for being silly about lipsticks and parties, but for being dishonest about her past and pretending Narnia never happened. Pretending to remember it as a game they used to play and that she had grown too old and mature for.
The donkey is a hopeful version of someone saved from the false-messiah-hood by not having really wanted it - unlike Antichrist in the Apocalypse. Whereas his friend the monkey is both false prophet and really wanting it. Like that of the Apocalypse.
Plato may be relevant for heaven. You see, Scipio had a dream, if you will take Cicero's word for it. In that dream, heaven and Plato's world of ideas are one and the same world. IT is not in the Bible, but it has not been condemned by Christianity and so there are a lot of Christians who think Plato and - on that occasion - the Cicero of Somnium Scipionis are right. Obviously C. S. Lewis is one, and so was probably Charles Williams.
(you would probably gather from A Horse and His Boy and The Last Battle that Islam was some kind of Satanic cult).
Not quite. First of all, the Calormene Empire owes as much to Assyria as to Ottoman Empire. Second of all the Calormene cult is polytheistic, with clear references to Hindooism - many armed idols and human sacrifice in the Thuggee cult. Third, C. S. Lewis is lampooning something much closer at home. The first mention of Calormenes is in The Voyage of the Dawn Treader. Eustace Clarence Scrubb feels they are the most serious and reliable people, the "least phoney" if I recall correctly.
They do human sacrifice in the capital, they do slave trade, they do forced marriages, they do ... organisation very well and intrigues even better.
But yes, part of what Islam has historically been about is some of these things. But Islam is just one of the English élite's non-Christian pets in major religions and - for that matter - non-Catholic pets in Christian confessions. Others are benighted with Hindooism (witness the Blavatskaja crew). Others like Eastern Orthodox better than Protestants and Catholics, hoping they will be more ... Mystic (in the sense they want). Or even Roman Catholics better than Protestants, as long as they keep thinking of Teilhard de Chardin and Küng and stuff and forget about Monseigneur Lefèbvre and Kolbe Institute (who are Roman Catholic Young Earth Creationists). Some English Protestants hated Rome so much that they ceased being Christian when finding that meant anything like Catholicism. C. S. Lewis was during his writing carreer, as Gilbert Keith Chesterton under an earlier part of his, Anglo-Catholic. Not Puritan, but Anglo-Catholic. Chesterton became Roman Catholic. He was, by the way, one generation earlier than C. S. Lewis. At any rate, CSL did not hate Rome to the extend of hating what he considered "common Christianity" in it. Some do.
Tash is inexorable like the "god" of Calvin and like the "life-force" of Atheist Calvinist (as Chesterton dubbed him) George Bernhard Shaw. So, "life-force" worshippers being anyway close to C. S. Lewis' definition of Satanist, and his having depicted Weston and Devine (non-oriental) as Satan worshippers in another context ... look at the non-Christian English élite (including some such Jews as do not take the Torah too seriously for that) and you will find exactly what Calormen is all about.
keep them away from The Voyage of the Dawn Treader.
I find the book very inspiring. As long as no clever set of smartasses (including Christian ones) believe they are Aslan, believe they have the right to punish Eustace Clarence Scrubb with unforeseeable and lawless "consequences" for what they consider his selfishness. In C. S. Lewis' own work, it is Aslan, i e Jesus Christ, i e God who decided that that particular gesture should be punished by getting into a sea where only chance of not drowning was adapting as ill or as well as he could to the crew of the Dawn Treader.
But neither a single man, nor a crew of men can take such a responsibility about the soul of a fellow man. Among Catholics, penances are individual things and they are openly set up for sins openly known. They are not secretly but inexorably imposed by secret judges and watchers. Not even for very bad sins. Still less for just having a very bad attitude.
St Valentine's Day
No, I do not think a sane man can attack C. S. Lewis if he is a Catholic or has similar ethics (whether he lives up to them or not). If he has not at all similar ethics, I am not sure one can call him sane.