First, I was able to make contact with the descendents of Max Einhorn, who were as pleasantly surprised to hear from me as I was to find them. Looks like some good stuff ahead there.
Second, I went back to JRI-Poland and did a search for "Surname like EINHORN and Given Name like NOACH" and found a birth record from Sokolow Malopolski for Sina Einhorn, b. 1861 to Noach and Taube. That matches the variants of Tovah that I've seen for the mother of Sarah, Chana, and Max, so I'm adding her to my tree.
I'm not familiar with the name "Sina", but I don't think that she's the same person as Sarah (b. 1864), because Sarah's birth year stayed very consistent across all her records. (ETA: Also, Sarah's granddaughter was given the middle name Zena, which I just realized is the same name as Sina.)