I am not sure exactly which schematic you are following. As I said in my first reply: only version 3.0 board design uses PCF8574 — in fact, it uses one on the top level board, and one on the driver board. Revisions 3.1 and 3.2 do NOT use PCF8574, instead, they use a single PCA9555 IO expander chip. The reason is that PCA9555 is a 16-channel IO expander chip, while PCF8574 is 8-channel. So the original 3.0 design had to use 2x PCF8574, and later with PCA9555 it only needs one single IO expander chip. Also, PCA9555 uses totem output so can source much higher current into the traics on the driver board. PCA8574, on the other hand, uses weak pullup so cannot source high current (and for that reason OS 3.0 driver design had to use additional PNP transistors to provide high current).