Your first answer is: Boil the lactose with your priming sugar and add it at bottling. Yeast are lactose intolerant
They won't eat that stuff. Another name for what you are making is Milk Stout and it is supposed to be mildly
You are correct on your second question. Add hot enough water to get up to 170. The problem is that you probably want to rinse the grains and you DO NOT want to rinse the grains with water any hotter than 170. You will get an astringent off-flavor like a spent tea-bag. I like to rinse with 165 degree water to avoid that.