^The problem is that you're starting with a desired conclusion -- that the "M" stood for something specific -- and making up convoluted rationalizations for it. You can use that kind of reasoning to "prove" anything, no matter how nonsensical. The human mind is very good at constructing patterns that don't exist. Which is why we have to be skeptical of any reasoning based only on that kind of patterning. Just because M could
stand for "Mars," that in no way constitutes proof that Roddenberry intended it to. I mean, hey, the word "man" appears a number of times in the pitch document, so that must "prove" that Class M meant a world where men could live! Or, look, here's the word "maximum," just a few lines above "Class M!" It must mean "maximum habitability!" You see how easily this kind of reasoning can lead you in any random direction?
Again, you need to remember what a slapdash document the format proposal was meant to be. It wasn't for publication; it was just something to hand out to network executives to support his verbal pitch for the show. It's riddled with typos and a lot of it is clearly handwaves and gibberish. He didn't specifically intend to make Spock half-Martian; he said "probably half-Martian" because he hadn't really given a lot of thought to the specifics yet and "Martian" was just the first thing anyone in the 1950s-60s thought of when they thought of creatures from outer space. This was a rough sketch of his ideas for Star Trek
, nothing more. It's very unwise to read too much meaning into whatever random coincidences might crop up in it.
EDIT: Hey, look what I found. The Planetary Habitability Laboratory
has actually proposed adopting the term "Class M" for Earthlike planets, using it to stand for "mesoplanet," i.e. a planet of moderate temperatures suitable for most Earthly organisms. They also propose Class P (psychroplanet) for cold terrestrial worlds (as Mandel does) and Class T (thermoplanet) for hot ones (unlike canon, which uses it for gas giants). They claim that the correspondences of M and P classes to the Trek scheme is a lucky accident, but they say it would help make the scheme accessible to the general public.