Hi there and thanks for your informative posting.
Sounds to me like the neck has been cleared so I would forget that in terms of a treatment approach for now as it's been done and you haven't progressed.
Sounds to me a lot more like a traction type issue, perhaps with some small subluxation in the shoulder joint as well during the action of pitching. This is very very common in people who throw balls at high speed e.g. baseball, cricket, javelin. It is also likely thew the 'injury' has a slow insidious onset meaning no one event caused it but rather that it was an accumulation of repetitive insults to the tissue in that area.
A key part in the identification of the issue and the rehabilitation would be to video analyze your pitching mechanics. Not just fresh but also towards the end of a long/typical stint on the mound. If the whole body is not connection from your stance leg through the hip/pelvis/lumbar spine/thoracic rotation etc etc then the arm will overwork/over-extend on the wind up. If all is not in order and if your external rotator cuff is not strong in eccentric loading then the slowing down to release of the ball will cause all sort of compensation in the glenohumeral (shoulder) joint. This can cause a traction type injury to the brachial plexus (nerves running down the arm.
Sorry if this sounds a bit complex but actually it is, especially for a baseball pitcher. You need to see a physio who works with pitchers. Perhaps look to a clinic that supports one of the big local teams and get them to check you out.
For reference: Craig Allingham is an excellent Aussie Physio who specializes in this type of rehabilitation although he is not based in the USA.
Best of luck