Qwertyuiop:
Hair maybe explained by him simulating human more after parting with Li Lun? Their split could also be earlier than 1/2 years before massacre as we don't see the face of the doctor Zhu Yan saved from Li Lun so he could have been much younger.
The 8 years was so jarring when they are more than 30 000 years old (and I think basically the same age) but that makes it even worse as they have been all their lives together...and since it was so fresh they hadn't had time to reconcile.
Good theory about the hair! My guess is basically the same, of Zhu Yan adapting to the mortal realm, but also because he’s away from Ying Lei’s grandpa (whose name escapes me right now!) who we know for a fact is the one who braided Zhu Yan and Ying Lei’s hair! Perhaps being away from the grandpa, no one took care to braid his hair anymore haha
Zhu Yan and Li Lun growing up together for thousands of years, only to be separated by a mad doctor huhuhu