Maybe I am stupid, but myositis means to my knowledge inflamation of muscle. Am I wrong in this? If not how is it possible to have myositis of the elbow?
In general passive mobilisation should be avoided as long as there is evidence of an uncontrolled bone growth since this might disturb healing process.
The radial nerve palsy has this come on gradually due to this uncontrolled bone growth or as a result of the fracture. this would be my major concern since it could develop into a permanent loss of lower arm function and in that case the mobility of the elbow is of little importance. So my choice would be to try to stimulate the healing process of the nerve by e.g. active arm movements, in which muscles innervated by the radial nerve are involved (e.g. PNF-patterns).