Hello,
About Grit, use of grit is because sometimes there could be obstruction of the digestive system (it is rare though). I use fine sand while they are small, but for ducks what is important is to never let water run out, because their digestion requires water to push down and that is the major factor to prevent obstructions.
I would just keep at 37.5 for now, but nothing guarantees us that the incubator itself is properly calibrated. Next incubation you verify the timing when they hatch and if too soon or too late you make further adjustments. All termometers shift in time.
About Rollers, can you have zones with rollers for pre-last 3 days and another one without rollers for the last 3 days? You can also choose to manually rotate the necessary ones, but opening and closing can create the shrink wrap effect on the hatching ones.
About buying eggs from France. I would have first run a test with cheap eggs to ensure your incubator runs fine and only after I would put money into expensive imported eggs, but it's your call. Also about consanguinity the idea of an import is to mix local and imported. That is how you cut consanguinity by combining different origins. By importing from France, that breeder possibly has multiple generations of siblings ("reputable" is just a trust concept).
As an idea if you are not aiming at purity you can cross Khakis with Runners (do some research but I don't think there's any problem with that, but do make some research)