What makes me eligible to participate in the contest,
what disqualifies me from the contest (aside from plagiarism or other form of cheating),
and what are the exact prices for winning particular contests (aside from fame and satisfaction);
also, while the rules for how participants are ranked are usually provided, there is no information about how many “winning” positions on the ranking there are.
The question about prizes was raised before, but it was over a year ago, and the answer could have changed, or might change in the future. It should be specified explicitly either for each challenge, or in a unified terms and conditions document.
Yes, it should be unified. FAQ are not rules, and most people don’t read it, as it’s not obligatory to read FAQ (unlike rules).
You can often find the specific rules for a contest on the page with problems. Those answer most relevant questions, or those specific for the given contest (since most are organized by a 3rd party and just hosted at this site). Any disqualification rules should be written there, or sent as a warning by the staff.
In general, there are no rules about “eligibility to participate” in an internet contest. The Internet is free for all
Points 3 and 4 are a serious problem. It’s really hard to find (I only found out accidentally), which is annoying, but I don’t like to code with prizes in mind, anyway