Tse's lines are actually torn, perhaps due to stress, perhaps genetic, as he also experimented with them, but not as strongly. I think you mean a bag under the eyes. Remember, Atachi never sleeps because he is always around. ... You can get tse bags with sleep deprivation.

Why does Itachi have a slash on his headband?

He wore a headband with a leaf symbol with a horizontal slash over it, signifying his “missing-nin” status, meaning that he had cut ties with his native village, the Hidden Leaf Village.

Why does Itachi poke Sasuke in the forehead?

So, yes, it was an affection thing, but it was also to try and protect Sasuke from Obito. It's a friendly gesture of Itachi poking Sasuke's forehead. With that, Itachi wants to let Sasuke know he still loves him after all. But there's a chance that Itachi gave him something like the last step of the Sharingan.