Gap between stairs and concrete wall

There is a concrete retaining wall and a staircase constructed from precast concrete block steps. The retaining wall is to be clad on the side facing the staircase with natural stone veneer panels (Muschelkalk limestone). The veneer will be installed using a flexible medium-bed adhesive.

Between the concrete block steps and the retaining wall, there is a joint approximately 20–30 mm wide, extending to roughly the full depth of the block steps. Below this joint is a layer of crushed stone, followed by the concrete bedding in which the steps are set.

If the stone veneer panels are installed resting on the block steps, they would slightly bridge and cover this joint.

Questions

1. How should the interface between the concrete block step and the lower edge of the natural stone veneer be detailed?
   For example: should the veneer be installed bearing on the block step, or should a separation joint be provided (e.g. defined clearance with elastic jointing using a natural stone–compatible silicone or sealant)?
   1. Which material is recommended for filling the existing gap between the block steps and the concrete retaining wall?

Any help is appreciated!