As it is a reinforced concrete structure, the sliding should not be a concern due to the self-weight of the structure; same goes for overturning moment. The consideration of the frost penetration will govern the depth, if applicable. IF there is SOG, it can also govern the minimum possible depth of the footing.
Why would you ask that? How can a retaining wall transfer any thrust to the foundation?
There is no need to split the structure of this size in three portions. What is dynamic stiffness? I have not come across this term before in relation to the structural design of a building.