To sort of get around this I usually only memorize 1 concentration spell for each level of spell, and then I arranged my hotbar so that each level of spell takes up 1 column with the Concentration spells all on the same row. This way before I cast another Concentration spell, I take a quick glance along the Concentration row so I can double-check if I'd be cancelling an existing spell.
If you just stick with questioning Anders on Karlachs side, he'll eventually crack and say something along the line of "enough" and admit to being Zariels servant. You don't even need to roll.
That's why I refuse to use any concentration skills. I even respecced Karlach to fighter because I always forgot to use rage and when I didn't I did something to cancel it lmao
