25th & 10th Anniversary Challenge

As far as Thamasa goes, for gameplay purposes it's easiest to switch out Locke. He's had a ton of required time in your party at that point in the game, and is leagues ahead of other characters for magic growth. He also has a storyline reason for leaving (going with Celes). Also, all of his lines in the Thamasa segment are super-generic, and can easily be attributed elsewhere by just changing the name of the person who said them. I did this in Divergent Paths when I swapped out Locke for Leo.