The killer was not on the lease, that was established. But it was also established that she had not fully moved out and had a hair brush and other items still in "her room" so some modicum of privacy should be given to that room as being hers. The hammer was found in a wastebasket in her room, so that privacy was extended by the judge to disallow the hammer. I seem to remember the uniform coming from the back room with the discovered hammer and it was not just lying by the bodies.
I believe that the killer had every intent to kill both the husband and the wife, she just didn't manage it - likely thought that the husband was dead when he was still hanging on.
As for a complete recovery, I think they showed pretty well that the husband had a degree of brain damage with memory loss &c.
Finally Maroun. I was convinced this was her "write out" but maybe just a foreshadow. I did not get the impression that she "sacked up" (as someone above put it) to do her job, but rather that she rationalized that bartering the baby, and also sell the baby off to higher paying clients showed Maroun that the killer was calculating, manipulative and not PTSD. So rationalization to enable her to fight the case, not realizing that this is job she signed on to do.