Here's my two cents. The step will add some stiffness, so you're probably right that there will be some portion of the upper slab that doesn't really put a lot of bearing load down on the soil. I don't think that will be enough moment or shear for you to worry about as long as the slab is...
Curious what the concern is here. They should be able to backfill behind the stub wall prior to pouring the upper slab. are you worried about the differential settlement between native soil and the backfill? I would think that to be negligible.
"Let me help you by not helping you."
You could look at adding an additional layer of concrete wall on the outside that would cover over the brick. the new wall could be designed to carry the full load without having to count on the strength of the existing elements. If you are increasing the load on the existing wall on the sides...
SEAOC has some design examples that you could follow if you have access to them. The most recent from SEAOC would be the 2021 IBC SEAOC Structural/Seismic Design Manual, Vol. 3. Be aware that the ACI318-19 has some new provisions for special concrete shear wall designs that differ quite a bit...
