Capitalization: When we combine proper nouns, we capitalize attributive words when they precede place-names, as in Lakes Erie and Ontario, but the opposite happens when the order is reversed: the Appalachian and Adirondack m ountains. When a term is used descriptively, as opposed to being an actual part of a proper noun, do not capitalize it, as in "The California deserts do not get as hot as the Sahara Desert."
Numbers: Grace always trumps pedantry. Don't let rule-mongering make your prose unreadable.
House Style
Some questions have no "true" answers, only competing standards used in different places. There are of course differences in spelling and punctuation in various countries, but "house style" refers to the choices about (mostly minor) matters that each publishing house sets on its own. Newspaper publishers, for instance, often use different rules than book publishers do. It's not a question of which is "right" or "wrong"; learn to suit your mechanics to the forum for which you're writing.
(See also: this and this.)