I understand now that the problem is really that the mesh motion solver (you are using multiSolidBodyMotionSolver does not define pointDisplacement in the same way as e.g. displacementLaplacian. Therefore, the adapter does not know how to read displacements in this class context.
Since you are trying to use overset, do you also need to use multiSolidBodyMotionSolver? If yes, then you would also need to modify our Displacement.C accordingly.
As far as I know, overset should work with displacementLaplacian (check again Derek Risseeuw’s master’s thesis for details, he had a moving flap there).