@TheEccentric - Welcome to our forum. I’m glad you’ve found helpful information here. That’s our goal to inform & educate our members & their caregivers about ES. 
You did a really nice job with the 3D conversion of your imaging. Your symptoms sound horrible. I’m sorry you’ve been dealing with them for so long & were gaslit & dismissed! I’m glad you now have a diagnosis so you can move forward.
There’s definitely significant IJV compression bilaterally. Your left IJV looks to be dominant based on it’s larger size but the right one looks much more flattened. I also see what you mean about your cervical spine as you’ve lost the lordotic curve. You can start working on restoring the curve with gentle physical therapy exercises & much patience once you’ve had your surgery(ies) & are somewhat healed.
You asked about things to help reduce symptoms besides head elevation & aspirin. The reason aspirin helped you is because it’s a mild blood thinner. A number of our vES members have had good head pressure reduction when they began taking a prescription blood thinner such as Brilinta, Plavix or Xarelto so that could be helpful for you since aspirin helped a bit. Creatine has been mentioned (5-10 mg/day as helping reduce brain fog. Here’s a link to a post @Jules wrote that may be helpful:
Would it be possible for you to put up right & left sagittal plane images. In the images you’ve posted I can’t tell how much of a role the C1 transverse processes are playing in your compression. The rotating pictures are too small for me to see details very well.
Regarding knowing prior to surgery how much C1 might need to be removed, even a doctor can’t tell you that for sure. We’ve had members who’ve had good results with just styloidectomies, but that’s when the styloids are the dominant force causing the compression. More often it seems that C1 needs to be shaved to some extent as well. Doctors who do vES surgeries have varying opinions regarding how much shaving is necessary i.e. some do the least amount they think will give the iJV enough space to open, whereas others are more aggressive so they do more extensive shaving of the TP of C1. That should be among the questions you ask when you’re consider whom to have do your surgery(ies). Checking for other sources of compression is also something a doctor should do whom you’re considering for surgery. There can be soft tissues also causing compression such as muscle, nerve, other blood vessels, scar or connective tissue, lymph nodes, etc. These aren’t always visible in imaging but the compression they’re causing can show as a secondary area of compression below the styloid/C1.
We have one doctor on our Doctors List for your country, but I don’t know if he’s operated on cases as complex as yours. It would be worthwhile having a consult with him though.