The careful way that she and Harry have woven the protections on the manor make it imperative that her cover is perfect. When she grabs him next suspicion will have to fall on the two of them. She'll make sure Harry's alibi is air tight but anything she does to insure one for herself puts her at too much of a risk. No, she'll have to bank on her reputation and the relationships with the other investigators to keep her out of suspicion. And making sure Draco can't for a second imagine her as the Ripper is critical.