Has this been working for you in game?
I haven't noticed an increase with 18 int so far.
Definitely working. My tank chanter has like 15 or 16 int can't remember at the moment, and the linger is over half of the duration. So running the chant that gives will/fort saves I have almost a constant +20.
I'm also not really noticing the problem with how fast chanters get their invocations. Granted I am playing on POTD, but I also wasn't having problems with on Hard either. I wonder if some of you got hit with that difficulty bug unknowingly.
On POTD I have noticed however that a tank chanter just cannot soak the damage that even a super sub-optimal fighter tank can. I don't have Eder spec'd for tanking at all, but I am using him as a tank and he has soaked nearly 2x as much endurance damage as my super-duper min-maxed chanter tank. It's ok though! Thematically I love chanters and the utility they bring is insane.
There's a difficulty bug? How do you know if you have it? I'm playing a chanter that's not too terribly optimized on normal, which the game describes as fairly difficult, but things just keep exploding on me and I'm having trouble building up to invocations until the very end of fights.
That may come down to your armor and the like then, or just not controlling your skills well, I'm still re-learning all this myself :/ Keeping guys knocked down, getting the right armor with good stats. I've going a tank chanter that can take a fair beating, unless everything focuses on him first, then he pops like a cork.