Earlier quoted context omitted.
What's so bad about Mere Christianity? It ranks among my top 5.
It's completely idiotic. Lord, Liar, or Lunatic? Um, lunatic, obviously. He dismisses atheism with a single sentence, and exhibits an astonishing level of ignorance. If you read it as a Christian, you might not notice the idiocy, but if you aren't a Christian, it's really really bad, so many assumptions, so many times something is glossed over as if "obviously so", when it's so not. Fucking terrible, and obviously so…
He would accept that answer (at least if we take the trilemma argument in isolation). The point is not that he is "Lord", but that good teacher is not one of the viable options[1]. You clearly haven't read him very carefully.
[1] Although good teacher whose teachings have been manipulated/corrupted would seem to offer a fourth option. But then what, if any, of his teachings could be reliably attributed to the man himself? So "corrupted good teacher" is practically equivalent to myth. We thus have Lord, Liar, Lunatic, or Legend. Any of those four would seem a rational position.