Some observations regarding Mollie Schiff immigration :
A) It is possible that she was never naturalized.
B) If her father was naturalized, she didn't have to file papers herself.
C) She married a US citizen so she automatically became US citizen.
Regarding the year she arrived, have in mind that census records are not a very reliable source. Especially the 1900 census, which gives the arrival year as 1890, is full of inaccuracies. According to her death certificate she arrived in 1893. The Chiene Schiff that arrived on Aug 1st, 1895 is most likely the person you are looking for. As you probably have noticed, her father and brother sailed, also, from Hamburg and arrived in late July 1895. Although there is some distance between Slutzk and Schweksne, it is possible that Mollie had relatives living there or it was a stop during her trip for the US.
Another detail I have noticed is that her mother's name in her death certificate is given as Sofia Cohen while in her marriage record as Sophie Rosafsky. Any explanation for this?