Those are the normal questions you would do after reading it. It isn't clear and it isn't meant to be clear, at least that is my opinion.
The child, well... it isn't clear whose child he was, I don't even know if the master was telling the truth, but it could be 50/50, Kanou/Master. Even so, Kanou didn't mind having sex with his assumed child in front of an agonizing (if not dead) man who was in love of him.
So much secrets and hidden feelings. That's what is left.
Although, it could be as KyoZaNa✿ said: Kanou intended to make the kid's skin burn and blister so he wouldn't be buyed.
And the phrase "you chose him" gets a lot more meaning than if he would have wrapped him up as usual.
That's why is nice to read other comments and interpretations, you can see the story from a different point of view (⌒▽⌒)
So, keep asking questions!
ah, thank you for clearing that up for me (⌒▽⌒)
I don't understand... Did the seme eventually will kill the child (since he let him burn)? Did seme actually love dark haired uke (he kissed him when he was dying)? And, why did he have sex with their child? I just don't get it...