I read somewhere to use yeast nutrient at the end of boil and one or two more times in the next two days with a high ABV beer. Look at wine or Mead makers, they always use it (or at least should). And I just brewed an 11% calculated Stout with 2 sessions of nutrient and kveik from my kveikstock...
Not sure if you were doing coffee in the bill or in fermenter, but should be in fermenter. I use whole beans do don't have to worry about grounds getting through. It takes a little more or a little longer ( I use about an ounce of whole beans for 24 hours and get plenty of flavor).