The Cairn version of enemies that have an advantage to a save, should not be written "+1 to saves". This is actually a disadvantage.
Advantage would be "-1 to saves" since you want the dice to be equal or lower than the target number.
The 5e version of the enemy has advantage against charm, so I believe that the Cairn one should also have an advantage?
I believe it's just unclear writing again.
Example:
Dark devotion: +1 to will saves
That's a bad thing.
I'm guessing what you mean is:
Dark devotion: +1 to will when rolling will saves
Easiest fix:
Dark devotion: -1 to will save