To add to all of these question, The pcf8574 datasheet sepcifies the max i2c input rise time to be 1μs but it does not specify a minimum rise time. i am also confused about the first figure on page 7 of this datasheet,
the first figure show the use of a 1kohm resistor?
I submit that you should only concern yourself with the measurable resistance from SDA & SCL to
5V . Use the scope on those two signals and compare to Nick Gammons screenshots but focus on
the resistance.
Also, you need to investigate the overvoltage . Where is that coming from ? (the arduino ?)
Yesterday I got the resistance up to 2.5k.. im probably going to bring to to around 5k. I put a decoupling cap on the pcf8574. So far u have not had an unexpected pin changes.
However I'm in the processes currently building another exact device. I will have time to put my oscillocope on the new one while hopefully the other one works in the mean time.