I did the same things a few seasons ago, but I don't think it was a dumb move. I fired my level 5 trainer to start season 10, hired a level1 trainer, and trained FT for 5 weeks straight. In the championship tiebreaker game that season, I shot 18-21 from the line and won by 3.
As far as whether to train FT or Stamina, I think they both help your team, but FT shooting seems to get much better each pop you get, especially at lower levels. But I agree with Superfly even more so. Help out the players you plan to hang onto.
Once I scored a basket that still makes me laugh.