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Wadim-
I'm not sure I understand. The title certainly seems to suggest that they are homework problems, and MO has a very explicit "no HW" policy. Even if they are difficult and interesting HW problems.
No criterion is resistant to sufficiently good attempts at breaking it.
(I write that having just yesterday endured the joys of the security measures at several international airports...)
You can effectively vote to make a post CW by leaving a comment clearly explaining why it should be CW and flagging the post for moderator attention. If there's a consensus and the reasoning is good, then a moderator can convert it to wiki. If there isn't a consensus, it may be worth starting a thread on meta. Implementing vote-to-wiki (0) is not possible for us, (1) is likely to lead to "CW-bullying", and (2) makes the system more complicated without a very good reason.
Question #29926 is different. Without knowing where it's from, there's no way to tell if it's Olympiad-level or considerably harder (e.g. on the level of Catalan's conjecture). Knowing that it could be solved by congruences, I would have voted to close it and suggested that the OP ask at artofproblemsolving.com, but I didn't have that information when I saw that question before it was answered.
On the other hand, the Malaysian questions both 1) look much more like Olympiad questions and 2) were clearly identified as such. We should use any information we have in determining whether a question is appropriate.
Question #29978 isn't great, either, but at least it could potentially have been an easy step that someone in their research was stuck on, whereas there was no possibility of that with the Malaysian questions.
"6th form" likely refers to a Malaysian math competition which is divided up based on grade, rather than problems which are routinely assigned in school. (It doesn't invalidate your point, but "school-level" is a slightly inaccurate label.)
Incidentally, you can also "vote to CW" by going in and editing the post, if you have enough rep to do so. After some finite number (8?) of edits by other people, posts automatically convert. Not that I think this is a good thing to do, but it is technically feasible.
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