
I didn't understand one thing, why did Ciel ask if Sebastian could lie to him? And why did Sebastian looked so sad?

Revealed on the Noah's Ark arc, there's a contract that says Sebastian must not lie to Ciel.
There must be something written or portrayed on the paper (or letter, or photo, I'm not sure) that makes Ciel thinks Sebastian is lying to him (that's unusual to see Ciel shocked when he saw the letters in the wall, so it could lead to a new mystery).
I don't know which part that made you think sebastian looks 'sad' tho.

Wait... How.do u guys know yamato is uke? Was there a implication or scene I missed???

http://www.mangago.zone/read-manga/aka_to_kuro_katsura_komachi/mf/c001/26/
The terms, "I want to hold you" is usually equivalent to "I want to have sex with you" and said from the guy or seme's point of view. Since this story has been from Yahagi's POV we assume he's talking. And plus, it just makes sense. Yamato is oozing uke vibes.
And in http://www.mangago.zone/read-manga/aka_to_kuro_katsura_komachi/mf/c002/34/ Yamato is basically inviting him.

Spoiler ranting:
I refuse to believer this manga is turning into SnK (attack on Titan).. I mean I know it has always been borderline SnK but noooooo Sensei don't kill one of the main characters just for blood and tears!!!
.... Σ(っ°Д °;)っ I mean he still isn't dead, we can't see that he actually got hit in any Pannell so.. let's wait for clearance
http://www.mangago.zone/read-manga/watashi_no_ookami_kun/bt/309320/Ch5/1/
The guy in the background reminds me of Haikyuu's Karasuno and Nekoma