The blockyness of the face looks like it might be cos theres not enough polys ther ewhen the dispalcwement was generatred.

If you adjust the UV's yes it will screw with the displacement (as the UV's are whats determining the position of the map). In Zbrush you can swap a models UV's not too sure about in Mudbox though.

For the crashing issue, not too sure but have you tried using the dispalcement map as a mental ray native .map format as it tends to run a lot better when using one of them.