1. I usually credit MU and FM flights to Delta because the miles don't expire and the distance based earn has been beneficial to me.

2. If there is a correlation between fare class and seat selection, that's news to me. I almost always get seats at the airport, and can't recall ever getting stuck with a bad seat.

3. The way fares are typically reported is discount from full fare e.g. 7折 = 30% off of Y. Various discount levels map to specific fare buckets. Base fares are tied to distance just like train fares. The difference is that discounting gives airlines quite a bit of flexibility. Pre-2000, this flexibility did not exist at all, and it has become increasingly complex since, but you'll still never pay more than full fare in any class.

4. I always check a bag (since flights are point to point, the odds of bags getting lost is quite low).

5. CZ is definitely bigger than CA and MU, but that's mainly because it's an amalgamation of ~5 different CAAC divisions.