I'm in the middle of studying this now and I think I can give some sort of input to your first question.
Rankine Active Pressure Coefficient, Ka=(1-sin (angle of internal friction))/(1+sin (angle of internal friction))
total stress = Ka*moist unit weight*height from ground surface to point of interest
moist unit weight= [(degree of saturation*void ratio+specific gravity)* unit weight of water]/(1 + void ratio)
water content = (degree of saturation*void ratio)/specific gravity
This looks so much better with the actual symbols but this is the connection that stood out to me.
ETA: So it seems that the pressure exerted by the water is part of the stress put on the soil particles which adds to the overall active pressure on a particular barrier like a retaining wall or on some other object.