1. Keep in mind that if you replace him you need to pay his salary. So if you think only financially wise, it wouldn't be a good idea to replace him.
Performance wise, the old level 9 trainer will perform as a level 8 with the same wage as the new level 9. So you need to trade-off. What do you want, save money or slightly better training?
2. It's random. No one can really tell when a 'x' trainer/doctor/PR comes.
3. Because the other guy demands more wage. They perform the same though.