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Maybe ACI code can help as under,

In my personal opinion, you have two options (I am not much aware of software options, other members may correct me) i) Use area element and generate varying load using joint pattern options in SAP. You may not get software generated design in this case. ii) Use dummy area elements with zero property modifiers. Then load these dummy area elements as per step 1 above. During analysis, loads will be transferred from dummy area elements (Having zero stiffness) to frame elements & you can get the software design for these frame elements. (Some engineers do it for wind load analysis on frame buildings, where wind loads applied on non-structural walls is transferred to columns by this method). Alternately, in STAAD there is a direct option of hydrostatic loading on frame elements. Snapshot below.

This grout is usually done at crown, that's why top 120 degree is chosen by the designer. However, you may realistically reduce this pressure intensity & also change the angle from obtuse to acute. My advice, conservatively neglect internal pressure (Empty condition).

You are right about non linear behavior of soil. However, in SAP one can model compression only behavior of soil.