Thanks for translating! Your traslation seems fine to me, even if it is going through two languages.
Five bucks says that Kate was smart and wrote the letter in soot, so she can just rewrite the contents on the fly. It's interesting to see how powerful everyone is with soot, the control that Louise can exert is especially concerning. And the star bearers seem to be aware of every type of soot manipulation as well.
My theory for why they are so many children and so few adults is that they are pruning off any shadows that can't develop soot manipulation, or integrate with their face in time. It seems that the shadows are also being used for some malicious purpose. Perhaps using soot harms the shadows in some way? We have yet to see the Grandfather use or give off any soot but the tiny amount they put in all the coffee.