1. If you use BuzzerManager then it will tell you the expected salary of your player the following season, so if you're training players it can avoid any salary shocks at the end of the season.
It depends on your player's listed position as to how much a skill affects their salary. BuzzerManager has a salary calculator you can play around with, for example you can take one of your current players and add 4 levels of rebounding to see what that does to his salary. There is this table so you can see the effect at a glance:
(160760.16).
Bigs tend to have much bigger salaries than guards/wings.
2. I think you will see better attendances against human teams, and if you lower ticket prices as you plan then that should help.
3. The Transfer Price Estimator is not always a useful tool, even when it does provide a value. I'd suggest going to the transfer list, put in the criteria for players similar to your own, and see what others are selling for.
4. If you reduce salary you should be ok. You don't necessarily have to sell your homegrown player Atkinson if you'd like to keep him, he'll provide a merchandise boost being homegrown and as he's probably reached his potential cap he won't train much further (and thus won't increase in salary). You could sell some other high salary players, then buy some cheap young outside players to train and switch training to something suitable for them.