I have seen cases where the tennis elbow was caused by Carpel tunnel. In America, most docs are pretty slow to recommend surgery for this, so when they do most options are usually exhausted.
Have you had a neck MRI? That would often be the next step if it has been treated without success, but the neck is still suspected. The only thing I dont see you trying already is eccentric exercise that starts within tolerance and progresses to an aggressive resistance. I think it's good to be thorough and make sure you have covered all bases. If it does come down to surgery as decided by you and the docs, at least it has a high success rate.
Have you had a neck MRI? That would often be the next step if it has been treated without success, but the neck is still suspected. The only thing I dont see you trying already is eccentric exercise that starts within tolerance and progresses to an aggressive resistance. I think it's good to be thorough and make sure you have covered all bases. If it does come down to surgery as decided by you and the docs, at least it has a high success rate.