welcome to TiMS
There are some people who have been treated for CCSVI (chronic cerebrospinal venous insufficiency) in other veins such as the jugulars and also had their May Thurner treated at the same time. Some of them have big improvements and some have mild and some have no improvements. But I don't know anyone who was treated only for May Thurner, as you're suggesting. Typically there are intraluminal abnormalities also found in the jugular veins, the azygous vein, or compression of the left renal vein. Jugular and azygous stenoses are typically treated by ballooning; May thurner and left renal vein nutcracker syndrome are typically treated by stenting, since these are compression syndromes, and a stent is able to withstand the compression coming from the nearby artery.
If you are seeing an IR for the May Thurner syndrome, then you could ask that IR about checking/treating the jugulars and azygous as well. Or you could go to one of the specialized CCSVI IRs who could treat everything including the May Thurner at the same time.
I had pretty big improvements for some of the symptoms you list after I had my bilateral jugular stenoses treated. Improvements in vision acuity, eye pain, ability to see brightness of colors. Also improvements in bladder frequency which I have had since I was 16! That didn't happen right away after the procedure but once it started improving it has lasted and I would call myself normal in that area which I have not been able to say for twenty years. My heat intolerance is improved but still present if it gets hot/humid enough. Balance is better, numbness is improved but still present in some ways.
As for how May thurner is associated with CCSVI, the idea with CCSVI is that the venous drainage from the brain and spine is slowed by blockages in the major veins (jugular, azygous). The iliac vein is not one of those major veins but a blockage in the iliac vein means that the flow that would be going through the iliac vein needs to be rerouted, and it may be rerouted into the cerebrospinal drainage veins. If there is also a blockage in the cerebrospinal veins, this additional inflow from the iliac vein will add to the flow that is slowed or refluxing because of the cerebrospinal veins blockage.
I've already written a lot but want to emphasize that CCSVI is in the discovery phase and there is some controversy as to whether partial blockages in the jugulars and azygous are harmful or harmless. My own jugulars were nearly completely blocked (99% blockage on one side, 77% blockage on the other) and personally I think that was harmful, especially considering some immediate and some gradual relief of symptoms after it was treated. CCSVI can be tested for by doppler ultrasound or by a more expensive MRI of the veins. Some insurance companies will pay for treatment. There is a lot of research indicating a strong association between MS and CCSVI although there is also research that failed to find that association.