First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
The symptoms and story that you present are complex and can not be fully addressed without an examination, but I will try to answer the specific questions you asked. First, all of your symptoms can not be explained in the cervical spine. The weakness and pain in the arms and shoulders is likely due to your cervical spine problems. However the tremor that affects your handwriting is a classic symptom of Parkinsons disease (along with shuffling gait, loss of facial expression, slowed movements, etc), but can also be seen in many different types of tremor. The "sciatic pain" that you described is unlikely to be caused in the cervical spine, and has numerous causes in the lumbo-sacral area. I would recommend that you get an EMG of the upper and lower extremity and a SSEP (somatosensory evoked potential) to assess the degree of problems that are coming from pinched nerves vs spinal cord compression. I would also recommend that you switch pain medications. Each person responds a differently, and if one is not working you should give others a try. Medications such as neurontin, elavil, effexor, cymbalta, tegretol, etc. have been shown to be effective in the type of pain that you describe. You should also be evaluated by a movement disorder specialist (neurologist that did further training in movement disorders) to assess whether you tremor may be early Parkinsons or a more benign tremor.
I hope this has been helpful.