For the most part, you must offer proof of your booking, including your ticket purchase confirmation, boarding pass, passport, and potential receipts. If you talked with the airline about the cancellation, you should also offer screen captures of what was discussed.

Yes, aside from the compensation you received from the airline for your troubles, you could also obtain it for other reasonable expenses. For instance, if you had to pay for meals, transportation, and accommodation that would have otherwise not been necessary, then you are entitled to receive payback for that as well.

Yes, most of the time, an operational issue can entitle you to receive flight cancellation compensation. The condition is that it has to be under the airline’s control. Some situations, such as a large bird being ingested in the motor, could be passed off as “extreme.” However, if the issue that appeared could have been easily prevented through maintenance, then you are likely eligible.

A denied claim does not have to be the end of the road for you. If you know you are right, you can take legal action and even take the matter to court. That said, you might get better results if you claim through a specialized company, as they could breeze through details you might not see.

The statute of limitation varies from country to country, most of them allowing around 3-4 years for you to file a claim (sometimes more). That said, to avoid any issues and get your money faster, you should file as early as possible.