Castiel says other than Gabriel the only other source of Archangel Grace on Earth is Lucifer. However they seem to be forgetting that the Archangel Michael (our universe's Michael) is still locked in the cage in hell. Rowena knows how to open the cage and with her newly enhanced powers it shouldn't be hard to for her to open the cage and bind Michael long enough to take some of his Grace. After-all Lucifer said that Michael couldn't deal with being locked up so he just went into a catatonic state, he said "Michael just sits there in the corner humming and touching himself." While our Michael's power is slightly greater than Lucifer's with the state he's in it would be easier to try and extract his Grace than to try and take on another Archangel who is at full power and in full possession of his mental faculties.
Maggie screams at the top of her lungs every time something goes "bump", like when her and Floyd are attacked in the forest or when Floyd accidentally trips, or when they see a vampire in the tunnel, which just draws the attention of even more monsters and possibly angels and demons. A person who finds themselves in a post-apocalypse style situation that requires being quiet and stealthy in order to survive would either learn to be quiet when startled/scared or die very early on. In this alternate universe it has been 8 years since the apocalypse, anyone still alive at this point would have the experience and the self-control to not scream like a frightened child every time something scares them.