[frodonk]

I saw some convincing arguments against living on trees at that time, but I thought it was best not to join in.

I brought up the idea of digging underground cities instead of building huge walls before, and it wasn't considered as a viable option, I consider living on trees an even less viable option :

then the person who was in the female titans was part official of the extramural troop?

or just a person who is infiltrated the troop... and has killed one member at a time and took his uniform

after her titan form was eaten, the person inside infiltrated the group by taking a survey corps cloak from one of the dead soldiers, if she was part of the group she would've been noticed right away, so she's somebody from the outside in my opinion.

as for the uniform, it would seem that the FT was a soldier.

Why did Erwin tell Levi that he should refill his gas cans and blades if Levi him self said he should have had enough?

he's psychic, no lol he anticipated that he'll need it when they retreat, and it looks like he's right. Levi was expecting his supplies to last without fighting another titan, erwin thought there might be more battles so he wanted levi to be fully equipped for the coming battle.