Frankly, even the button to open the secret room in Kellogg's house doesn't exist if you haven't talked to Nick. That's some hard core railroading.

That said, as Sandboxy as even Morrowind was, it had things you had to go through the main quest for. Oblivion locked away Sancre Tor, Skyrim had Sky Haven Temple and the top of Throat of the World. Fallout 3 had Raven Rock. New Vegas locked away Caesar's Fort. Even the most open games lock things away.

The Pitt is probably my second favorite of F3's DLC. Broken Steel takes so spot naturally, but Pitt is actually interesting with an unclear choice, and no clear moral right. One side is keeping slaves, one side is attempting to kidnap a child.

I think she's immune because she actually survived to birth. It would make sense that something that sickens adults as bad as the Pitt does, would kill any infant not immune.