A lot has been changed from the novel which is understandable to make it into 34 episodes. I am currently rereading the novel and noticed a lot of information and interactions ended-up missing but are still needed to fully understand.
- So far for the grey hair I think excessive sorrow as he had become living proof of Murphy's law (losing his uncle and all maternal family dying with traitor status, followed by losing his mother, being betrayed by his own father, loing his paternal family's support, losing his heir status his official position and the trust and support from the Emperor and imperal family, ........ ) I could go on but it's already a lot for a teen right? he could also have just been poisoned, but I believe this is the right track as in episode 10 I think Dou 4 found grey hair.
- for his lack of memory, I am not sure, it could be the poison or he might have been too weak his body too battered at the moment of his death, or it could be because the memories were too traumatic and are just blocked somewhere in his mind.
it could also be the fact that he was just a passenger Shou Gou was the first to go through the mirror as he was protecting her from the arrow
- For the kids and song Mo : ShouGou had Three kids in the novel, but she was only close with her daughter because she was too busy before and couldn't take care of the boys. They ended up being distanced. In the Novel she is particularly indulgent toward the duke of Wei (he was't as bad as in the show just weak and self indulgent) and because of their kids. She wonders if she should get back with him because of them but finally abandons the idea and decides that she should marry no one and be free. As attracted she is to Song Mo she fears she will loose control of her fate as his spouse so refuses him in the beginning. I think this is true in the show too.
- The killer's identity : It is a new trope that did not exist in the novel, but I feel the tattoo on the hand should be a major identifier right?