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Is there a list of all court cases related to EU 261/2004 that would help travelers evaluate their particular scenario?
Yes, at least of all court cases in the EU court system. Cases in national courts are not included. It is found by selecting "document information" on the page you linked to in the question. there is a long list under the heading "affected by case." There is also a summary of CJEU judgements on the official EU website.
But this isn't necessarily going to be particularly helpful.
How can one know if their particular reason for delay/cancellation counts as "extraordinary circumstances"?
You can't, really, unless the circumstances of your delay match those in one of the cases very closely. But there are some general principles that are outlined fairly well in an answer to a recent question, based on a pretty good blog post that appears to be from a company that handles these claims in return for a percentage of the payment (I haven't looked very closely). I know nothing about the company, but the post is very clear and thorough.
The general points are these:
You are entitled to receive evidence of the circumstances that they are claiming were extraordinary and the steps they have taken to deal with them. The airline's own statement of the circumstances is not sufficient by itself.
Circumstances are extraordinary if an event "is not inherent in the normal exercise of the activity of the air carrier and is beyond the actual control of that carrier on account of its nature or origin."