Why couldn't the scouts sneak into Wall Maria to access the basement?

I don't understand why they had to fully seal the wall to access the basement? I mean, instead of going full force and marching a whole ass army, why couldn't they just be covert?

would it not work to only send a small group, like maybe only Levi, Erwin, Eren, Hange, Mikasa, Armin, Jean, Connie and Sasha?

they've successfully snuck around when it came to the whole ordeal about the government coup, Erwin's arrest and in the kidnapping of Eren and Historia... so why not just have the core group sneak around and exit without ever alerting the enemy?

and push comes to shove, if they need to fight, they're about the only ones that stand a solid chance.