There are only two kinds of documents in current usage that spell out the year -- legal documents, and wedding invitations.
Legal documents normally spell the year in lowercase, such as in this numbing bit of prose from West Virginia:
For any tax imposed under the provisions of this article with respect
to any taxable year prior to the first day of January, one thousand
nine hundred eighty-three, a resident individual shall be allowed a
West Virginia exemption of six hundred dollars for each exemption for
which he is entitled to a deduction for the taxable year for federal
income tax purposes.
Some legal documents capitalize everything for extra pomp, but it's uncommon:
In testimony whereof, I hereunto subscribe my hand and affix the seal
of said Court, at Office, in Nashville, the 6th day of December in the
Year One Thousand Nine Hundred Eighty-Four and in the 209th Year of
American Independence.
Wedding invitations, as noted, tend to capitalize the first letter only; however, this seems to be the only justification I can find:
The “T” in Two thousand doesn’t have to be capitalized but everyone
does it so it would probably look incorrect if it wasn’t and it will
look more polished if it is capitalized.
So, in modern usage, it appears that the rules for capitalization are:
- Spell the year out in lowercase.
- Except in wedding invitations, where the first letter is capitalized because everyone does it.
There are no special rules for rarely used prepositions. According to the Chicago Manual of Style, all prepositions should be lowercased (except when they are stressed or occur as the first or last word). But I hardly ever see this style used. The AP style is much more common. According to the AP stylebook all prepostions containing three letters or fewer should be lowercased, while the Wikipedia Manual of Style says all prepositions containing four letters or fewer should be lowercased.
However, the Wikipedia rules are not enforced, and as a result the capitalization on Wikipedia is a mess, and often in violation of the style manual (as you had observed). I think the reason for this is that the AP rules are dominant in real life, so this is what people are used to (i.e., what “looks right”). This explains many of the cases that you listed: down, over, unto, upon etc. are all capitalized according to AP style.
So regarding your examples, it depends on the style guide you choose whether the four-letter prepositions should be lowercased. Three- or two-letter prepositions like on, off, out or up should always be lowercased (if they are really used as prepositions and do not occur as first or last word). Yet should also be lowercased, if used as a conjunction.
There is no difference between book titles and song titles. The AP style for instance says that its composition title guidelines apply to book titles, movie titles, opera titles, album and song titles, radio and television program titles, and several more.
Best Answer
The reason that Mohave Desert is capitalized more often than Sahara Desert seems to be a difference between practice in the U.K. and the U.S. See this Ngram, which shows that Americans are much more likely to use capitalization in "Sahara Desert" than Brits are. And clearly, Americans are also much more likely to mention the Mohave.
Both capital and lowercase are used, but you should pick one style and be consistent. There is absolutely no reason to be inconsistent, and treat the Mohave differently from the Sahara.
I don't know what the practice is in Australia, but from the example of "Daintree Rainforest", I would expect capitalization to be the usual form, as well.