I’m so sorry that you’re not seeing improvements after your surgery, it’s so rough for you 
Some people have had a resection of the digastric muscle during surgery if it’s causing compressions, it can compress the IJV, which is next to the vagus nerve, so could explain some of your symptoms. Some had the posterior belly removed, I think was the term- here’s a link to a discussion:
Article about jugular vein compression - General - Living with Eagle
Some members have tried dry needling into tense muscles, muscle relaxant meds or botox- I’m really sorry but I can’t remember on your journey what you’ve tried? The SCM muscle & omahyoid can cause issues & compressions too. I’m glad that you’ve narrowed down what might be contributing to your symptoms, it might be worth looking into a physiotherapist & whether there are any exercises which might help a bit? Also I just looked up TENS machines, & they can help with relaxing muscles, and can be used on the neck, so maybe that’s worth a try?
I hope others can help with suggestions as well!