Hi Brokk

You are obviously quite knowledgeable about you'd anatomy. However I think it would save you a lot of time and avoid the risk of a wrong diagnosis if you actually got it examined a good MSK physio.

...however one thing that sound quite possible you may have developed trigger points most likely in the left levator scapulae. Sounds about the right place and would get triggered by overhead shrugs. Upper trap rhomboids or something like post scalene could also be an issue.