Although you can use a mesh of plates of model the wall, I think keeping things simple by modeling the wall using beam element is good too. You can analyze for a foot length of the wall. Apply the soil pressure using Linear Varying member loads on the beam element.
Active pressure will always be less than passive pressure but I think it would be very conservative to design for passive pressure unless you see a real possibility of the wall pushing into the soil. If you are not comfortable going with the active earth pressure you can always go with the at-rest value. You may consult a geotech and get his recommendation on this.
I think you should do sliding check.