1. It was operational due to oversold Y cabin
2. The remark is probably because it's a saver fare, they can't be upgraded with miles before check in. So it's indicating that a reward upgrade is only possible at check in. Unrelated to you heibe upgraded.
3. Chances are always slim and depend on the cabin being oversold. As a Blue member in a Saver fare you're not very high on the priority list but on routes like BKK with a lot of leisure travel there is still a chance.
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Typing on phone so can't remember ecave order of questions but the above should roughly cover it